Category Archives: Shooting Tips

log is often a poor choice for low light.

UPDATE: Following much debate and discussion in the comments section and on my Facebook feed I think one thing that has become clear is an important factor in this subject is the required end contrast. If you take S-Log3 which has a raised shadow range and shoot with it in low light you will gain a low contrast image. If you choose to keep the image low contrast then there is no accentuation of the recorded noise in post and this can bring an acceptable and useable result. However if you need to grade the S-log3 to gain the same contrast as a dedicated high contrast gamma such as 709, then the lack of recorded data can make the image become coarser than it would be if recorded by a narrow range gamma. Furthermore many other factors come into play such as how noisy the camera is, the codec used, bit depth etc. So at the end of the day my recommendation is to not assume log will be better, but to test both log and standard gammas in similar conditions to those you will be shooting in.

Log gamma curves are designed for one thing and one thing only, to extend the dynamic range that can be recorded. In order to be able to record that greater dynamic range all kinds of compromises are being made.

Lets look at a few facts.

Amount of picture information:
The amount of picture information that you can record, i.e. the amount of image samples, shades or data points is not determined by the gamma curve. It is determined by the recording format or recording codec. For example a 10 bit codec can store up to 1023 shades or code values while an 8 bit codec can record up to 255 shades or code values (in practice this is a maximum of 235 shades as 16 are used for sync). It doesn’t matter which gamma curve you use, the 10 bit codec will contain more usable picture information than the 8 bit codec. The 10 bit picture will have over 1000 shades while the 8 bit one less than 255. For low light more “bits” is always going to be better than less as noise can be recorded more faithfully. If noise is recorded with only a few shades or code values it will look coarse and ugly compared to noise recorded with lots more levels which will look smoother.

Bottom line though is that no matter what gamma curve, the  maximum amount of picture information is determined by the codec or recording format. It’s a bit of a myth that log gives you more data for post, it does not, it gives you a broader range.

Log extends the dynamic range: This is the one thing that log is best know for. Extending the dynamic range, but this does not mean we have more picture information, all it means is we have a broader range. So instead of say a 6 or 7 stop range we have a 14 stop range. That range increase is not just an increase in highlight range but also a corresponding increase in shadow range. A typical rec-709 camera can “see” about 3 or 4 stops below middle grey before the image is deemed to be too noisy and any shades or tone blend into one. An S-log2 or S-log3 camera can see about 8 stops below middle grey before there is nothing else to see but noise. However the lower 2 or 3 stops of this extended range really are very noisy and it’s questionable as to how useful they really are.

Imagine you are shooting a row of buildings (each building representing a few stops of dynamic range). Think of standard gammas as a standard 50mm lens. It will give you a great image but it won’t be very wide, you might only get one or two buildings into the shot, but you will have a ton of detail of those buildings.

Hong-Kong-50mm log is often a poor choice for low light.
Shot of buildings taken with standard lens, think “standard gamma”

Think of a wide dynamic range gamma such as S-log as a wide angle lens. It will give you a much wider image taking in several buildings and assuming the lens is of similar quality to the 50mm lens, the captured pictures will appear to be of similar quality. But although you have a wider view the level of detail for each building will be reduced. You have a wider range, but each individual building has less detail

Hong-Kong-20mm log is often a poor choice for low light.
Buildings shot with 20 mm wide lens. Think “wide gamma” or log gamma.

But what if in your final scene you are only going to show one or two buildings and they need to fill the frame? If you shoot with the wide lens you will need to blow the image up in post to the show just the buildings you want. Blowing an image up like this results in a lower quality image. The standard lens image however won’t need to be blown up, so it will look better. Log is just the same. While you do start off with a wider range (which may indeed be highly beneficial) each element or range of shades within that range has less data than if we had shot with a narrower gamma.

Hong-Kong-20mm-cropped log is often a poor choice for low light.
Wide lens (think wide gamma) cropped to match standard lens (think standard gamma). Note the loss of quality compared to starting with standard.

Using log in low light is the equivalent of using a wide angle lens to shoot a row of buildings where you can actually only see a few of the buildings, the others being invisible and then blowing up that image to fill the frame. The reality is you would be better off using the standard lens and filing the frame with the few visible building, thus saving the need to blow up the image.

Hong-Kong-50mm-wasted-data log is often a poor choice for low light.
Shooting a scene where most of it is dark with wide lens (wide gamma/log) wastes a lot of data.
Hong-Kong-50mm-dark2 log is often a poor choice for low light.
Using a narrower lens (narrow or standard gamma) wastes less data and the information that is captured is of higher quality.

S-Log2/3 has a higher base ISO: On a Sony camera this higher ISO value is actually very miss-leading because the camera isn’t actually any more sensitive in log. The camera is still at 0dB gain, even though it is being rated at a higher ISO. The higher ISO rating is there to offset an external light meter to give you the darker recording levels normally used for log. Remember a white card is recorded at 90% with standard gammas, but only 60% with log. When you change the ISO setting upwards on a light meter it will tell you to close down the aperture on the camera, that then results in the correct darker log exposure.

S-Log3 may appear at first brighter than standard gammas when you switch to it. This is because it raises the very bottom of the log curve and puts more data into the shadows. But the brighter parts of the image will be no brighter than with a camera with standard gammas at 0db gain. This extra shadow data may be beneficial for some low light situations, so if  you are going to use log in low light S-Log3 is superior to S-Log2.

If you can’t get the correct exposure with log, don’t use it! Basically if you can’t get the correct exposure without adding gain or increasing the ISO don’t use log. If you can’t get your midrange up where it’s supposed to be then you are wasting data. You are not filling your codec or recording format so a lot of data available for picture information is being wasted. Also consider that because each stop is recorded with less data with log not only is the picture information a bit coarser but so too is any noise. If you really are struggling for light, your image is likely to be a bit dark and thus have a lot of noisy and coarse noise is not nice. Log has very little data allocated to the shadows in order to free up data for the highlights because one of the key features of log is the excellent way it handles highlights as a result an under exposed log image is going to lack even more data. So never under expose log.

S-log-levels log is often a poor choice for low light.
Chart showing S-Log2 and S-Log3 plotted against f-stops and code values. Note how little data there is for each of the darker stops, the best data is above middle grey.

Think of log as the opposite of standard gammas. With standard gammas you always try never to over expose and often being very slightly under exposed is good. But log must never be under exposed, there is not enough data in the shadows to cope with under exposure. Meanwhile log has more data in the highlights, so is very happy to be a little over exposed.

My rule of thumb is quite simple. If I can’t fully expose log at the base sensitivity I don’t use it. I will drop down to a cinegamma or hypergamma. If I can’t correctly expose the hypergamma or cinegamma then I drop down to standard gamma, rec-709.

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Video Camera Noise, ISO and Sensitivity.

It’s amazing how poorly this is understood. I’m also rather surprised at some peoples expectations when it comes to noise in shadow areas of video images.

First of all, all video camera sensors produce noise. There will always be noise to some degree and this is typically most visible in the darker parts of the image because if your actual image brightness is 5% and your noise is 5% the noise is as big as the desired signal. In the highlights the same noise is still there, but when the brightness is 80% and the noise is 5% the noise is much, much less obvious.

ISO: What is ISO with a video camera? On it’s own it’s actually a fairly meaningless term. Why? Well because a camera manufacturer can declare more or less any ISO they choose as the cameras sensitivity. There is no set standard. It’s up to the camera manufacturer to pick an ISO number that gives a reasonably bright image with an acceptable amount of noise. But what is acceptable noise? Again there is no standard, so ISO ratings should be ignored unless you also know what the signal to noise ratio is at that ISO. For decades video camera sensitivity was rated in db. The sensitivity  is measured at 0db in terms of the aperture needed to correctly expose a 90% white card  at 2000 lux. This is very precise and easily repeatable. The signal to noise ratio is then also measured at the unity (0db) gain point and from this you can actually get a really good understanding of how that camera will perform, not just sensitivity, but more importantly how much noise at the nominal native sensitivity.

But now, because it’s fashionable and makes us sound like film camera operators it’s all about ISO. But ISO on it’s own doesn’t really tell us anything useful. Take a Sony FS7 or F5. In standard gamma at 0db the ISO rating is 800 ISO. But when you switch to S-Log it becomes 2000 ISO (but you are still at 0db). Have you ever noticed that the image doesn’t get brighter even though you are increasing the ISO? The ISO is increased because what is actually happens is that you gain the ability to record a little over 1 stop further into the shadows as you are now using more of the sensors low range (which is normally well below the black level chosen for 709) with the side effect of also seeing a little more than twice as much more noise (1 stop = 6db = double). The camera isn’t actually becoming any noisier, but because your using a lower sensor range you will see more noise in the shadows, noise that in normal gammas goes unseen. It’s really important that you understand this as it explains why S-log looks very noisy in the deepest shadows compared to standard gammas.

Native sensitivity… again this is open to a bit of wriggle room by the camera manufacturer. With a camera shooting log, generally it is a case of mapping the entire sensor capture range from black to white to the zero to 100% recording range. Normally this is done using as little gain as possible as gain adds noise. But as noise reduction processes get better, including on sensor noise reduction, camera manufacturers have some space to move the mapping of the sensor to the recording up and down a bit. Sadly or us, high ISO’s sell cameras. So camera manufacturer’s like to have cameras with high ISO’s because people look at the ISO rating, but ignore the signal to noise figure. The end result is cameras with high ISO’s (because it sounds cool) but with less than optimum signal to noise ratios. It would probably be better for all of us if we started paying much more attention to the signal to noise ratios of cameras, not just the ISO. That may help prevent manufacturers from bring out cameras with ridiculously high native ISO’s that are noisy and frankly far from what we need, which is a good low noise base sensitivity.

The next issue is that people appear to expect to be able to magically pull something out of nothing. If you have areas of deep shadow in your image you can’t magically pull out details and textures from those areas without significantly increasing the noise in those parts of the picture. You can’t do it and you shouldn’t be trying to do it. If you have an 8 bit camera the noise in the shadows will be really coarse, you try to stretch those levels, even by a tiny bit, it’s going to get ugly fast (the same with 12 bit linear raw too). What’s the answer…. LIGHT IT PROPERLY OR EXPOSE IT BRIGHTER.

We appear to have lost the ability to light or expose properly. If you want detail in your shadows either expose them brighter or throw some light in there, then take the levels down in post. Remember it’s all about contrast ratios. Faces are normally 1.5 stops above middle grey and 3.5 stops above our dark shadow range. So if you want a lot of textures in your deep shadows expose the entire scene brighter, not just the foreground but the background and shadows too. If you expose faces at +4.5 above black. Mid grey will still be -1.5 stops below those skin tones and your shadows will still be 3.5 stops below your faces. the contrast ratio remains the same if you increase the overall light level, so now everything will be 1 stop brighter. Then take the levels down by 1 stop in post and bingo, you noise levels are cut in half and your shadows look so much better and might actually now contain some useable picture information.

To follow on from this I recommend reading this: http://www.xdcam-user.com/2015/03/what-is-exposure/

Deeper Understanding Of Log Gamma. Experiments with a Waveform Display.

I started writing this as an explanation of why I often choose not to use log for low light. But instead it’s ended up as an experiment you can try for yourself if you have a waveform monitor that will hopefully allow you to better understand the differences between log and standard gamma. Get a waveform display hooked up to your log camera and try this for yourself.

S-Log and other log gammas are wonderful things, but they are not the be-all and end-all of video gammas. They are designed for one specific purpose and that is to give cameras using conventional YCbCr or RGB recording methods the ability to record the greatest possible dynamic range with a limited amount of data, as a result there are some compromises made when using log. Unlike conventional gammas with a knee or gammas such as hypergammas and cinegammas, log gammas do not normally have any highlight roll off, but do have a shadow roll-off. Once you get above middle grey log gammas normally record every stop with almost exactly the same amount of data, right up to the clipping point where they hard clip. Below middle grey there is a roll off of data per stop as you go down towards the black clip point (as there is naturally less information in the shadows this is expected). So in many respects log gammas are almost the reverse of standard gammas. The highlight roll off that you may believe that you see with log is often just the natural way that real world highlights roll off anyway, after all there isn’t an infinite amount of light floating around (thank goodness). Or that apparent roll off is simply a display or LUT limitation.

An experiment for you to try.

xdcam-greyscale-300x169 Deeper Understanding Of Log Gamma. Experiments with a Waveform Display.
Click on the chart to go to larger versions that you can download. Display it full screen on your computer and use it as a test chart. You may need to de-focus the camera slightly to avoid aliasing from the screens pixels.

If you have a waveform display and a grey scale chart you can actually see this behaviour. If you don’t have a chart use the grey scale posted here full screen on your computer monitor. Start with a conventional gamma, preferably REC-709. Point the camera at the chart and gradually open up the aperture. With normal gammas as you open the aperture you will see the steps between each grey bar open up and the steps spread apart until you reach the knee point, typically at 90% (assuming the knee is ON which is the default for most cameras).  Once you hit the knee all those steps rapidly squash back together again.

What you are seeing on the waveform is conventional gamma behaviour where for each stop you go up in exposure you almost double the amount of data recorded, thus capturing the real world very accurately (although only within a limited range). Once you hit the knee everything is compressed together to increase the dynamic range using only a very small recording range, leaving the shadows and all important mid range well recorded. It’s this highlight compression that gives video the “video look”, washed out highlights with no contrast that look electronic.

If you repeat the same exercise with a hypergamma or cinegamma once again in the lower and mid range you will see the steps stretch apart on the waveform as you increase the exposure. But once you get to about 65-70% they stop stretching apart and now start to squeeze together. This is the highlight roll off of the hypergamma/cinegamma doing it’s thing. Once again compressing the highlights to get a greater dynamic range but doing this in a progressive gradual manner that tends to look much nicer than the hard knee. Even though this does look better than 709 + Knee in the vast majority of cases, we are still compressing the highlights, still throwing away a lot of data or highlight picture information that can never be recovered in post production no matter what you do.

Conventional video = Protect Your Highlights.

So in the conventional video world we are taught as cameramen to “protect the highlights”. Never overexpose because it looks bad and even grading won’t help a lot. If anything we will often err on the side of caution and expose a little low to avoid highlight issues. If you are using a Hypergamma or Cinegamma you really need to be careful with skin tones to keep them below that 65-70% beginning of the highlight roll off.

Now repeat the same experiment with Slog2 or S-log3. S-log2 is best for the experiment as it shows what is going on most clearly. Before you do it though mark middle grey on your waveform display with a piece of tape or similar. Middle grey for S-log2 is 32% (41% for S-log3).

Now open up the aperture and watch those steps between the grey scale bars. Below middle grey, as with the standard gammas you will see the gap between each bar open up. But take careful note of what happens above middle grey. Once you get above middle grey and all the way to the clip point the gap between each step remains the same.

So what’s happening now?

Well this is the S-log curve recording each stop above middle grey with the same amount of data. In addition there is NO highlight roll off. Even the very brightest step just below clipping will be same size as the one just above middle grey. In practice what this means is that it doesn’t make a great deal of difference where you expose for example skin tones, provided they are above middle grey and below clipping. After grading it will look more or less the same. In addition it means that that very brightest stop contains a lot of great, useable picture information. Compare that to Rec-709 or the Cinegammas/Hypergammas where the brightest  stops are all squashed together and contain almost no contrast or picture information.

Now add in to the equation what is going on in the shadows. Log has less data in the shadows than standard gammas because you are recording a greater overall dynamic range, so each stop is recorded with overall less data.

Standard Gammas = More shadow data per stop, much less highlight data = Need to protect highlights.

Log= Less shadow data per stop, much more highlight data = Need to protect shadows.

Hopefully now you can see that with S-log we need to flip the way we shoot from protecting highlights to protecting shadows. When you shoot with conventional gammas most people expose so the mid range is OK, then take a look at the highlights to make sure they are not too bright and largely ignore whats going on in the shadows. With Log you need to do the opposite. Expose the mid range and then check the shadows to make sure they are not too dark. You can ignore the highlights.

Yes, thats’ right, when shooting log: IGNORE the highlights!

WF-Cinegamma-3-1024x576 Deeper Understanding Of Log Gamma. Experiments with a Waveform Display.
Cinegamma highlight roll off. Note how the tree branches in the highlights look strangled and ugly due to the lack of highlight data, hence “protect your highlights”.
WF-Slog-graded-1024x576 Deeper Understanding Of Log Gamma. Experiments with a Waveform Display.
Graded S-Log2. Note how nice the same tree branches look because there is a lot of data in the highlights, but the shadows are a little crunchy. Hence: protect your shadows.

For a start you monitor or viewfinder isn’t going to be able to accurately reproduce the highlights as bright as they are . So typically they will look a lot more over exposed than they really are. In addition there is a ton of data in those highlights that you will be able to extract in the grade. But most importantly if you do underexpose your mid range will suffer, it will get noisy and your shadows will look terrible because there will be no data to work with.

When I shoot with log I always over expose by at least 1 stop above the manufacturer recommended levels. If you are using S-log2 or S-log3 that can be achieved by setting zebras to 70% and then checking that you are JUST starting to see zebras on something white in your shot such as a white shirt or piece of paper. If your camera has CineEI use an EI that is half of the cameras native ISO (I use 1000 or 800 EI for my FS7 or F5).

I hope these experiments with a grey scale and waveform help you understand what is going on with you gamma curves. One thing I will add is that while controlled over exposure is beneficial it can lead to some issues with grading. That’s because most LUT’s are designed for “correct” exposure so will typically look over exposed. Another issue is that if you simply reduce the gain level in post to compensate than the graded footage looks flat and washed out. This is because you are applying a linear correction to log footage. Fo a long tome I struggled to get pleasing results from over exposed log footage. The secret is to either use LUT’s that are offset to compensate for over exposure or to de-log the footage prior to grading using an S-Curve. I’ll cover both of these in a later article.

S-log-levels Deeper Understanding Of Log Gamma. Experiments with a Waveform Display.
Chart showing S-Log2 and S-Log3 plotted against f-stops and code values.

 

 

 

 

 

 

 

 

 

What about shooting in low light?

OK, now lets imagine we are shooting a dark or low light scene. It’s dark enough that even if we open the aperture all the way the brightest parts of the scene (ignoring things like street lights) do not reach clipping (92% with S-Log3 or 109% with S-Log2). This means two things. 1: The scene has a dynamic range less than 14 stops and 2: We are not utilising all of the recording data available to us. We are wasting data.

Log exposed so that the scene fills the entire curve puts around 100 code values (or luma shades) per stop above middle grey for S-log2 and 75 code values for S-Log3 with a 10 bit codec. If your codec is only 8 bit then that becomes 25 for S-log2 and 19 code values for S-Log3. And that’s ONLY if you are recording a signal that fills the full range from black clip to white clip.

3 stops below middle grey there is very little data, about thirty 10 bit code values for S-Log2 and about 45 for S-log3. Once again if the codec is 8 bit you have much less, about 7 for S-Log2 and about 11 for S-log2. As a result the darker parts of your recorded scene will be recorded with very little data and very few shades. This impacts how much you can grade the image in post as there is very little picture information in the darker parts of the shot and noise tends to look quite coarse as it is only recorded with a limited number of steps or levels (this is particularly true of 8 bit codecs and an area where 8 bit recordings can be problematic).

So what happens if we use a standard gamma curve?

Lets say we now shoot the same scene with a standard gamma curve, perhaps REC-709. One point to note with Sony cameras like the FS5, FS7, F5/F55 etc is that the standard gammas normally have a native ISO one to two stops lower than S-Log. That’s because the standard gammas ignore the darkest couple of stops that are recorded when in log. After all there is very little really useable picture information down there in all the noise.

Now our limited dynamic range scene will be filling much more of our recording range. So straight away we have more data per stop because we are utilising a bigger portion of the recording range. In addition because our recorded levels will be higher in our recording range there will be more data per stop, typically double the data especially in the darker parts of the recorded image. This means than any noise is recorded more accurately which results in smoother looking noise. It also means there is more data available for any post production manipulation.

But what about those dark scenes with problem highlights such as street lights?

This an area where Cinegammas or Hypergammas are very useful. The problem highlights like strret lights normally only make up a very small part of your your overall scene. So unless you are shooting for HDR display it’s a huge waste to use S-log just to bring some highlights into range as you make big compromises to the rest of the image and you’ll never be able to show them accurately in the finished image anyway as they will exceed the dynamic range of the TV display.  Instead for these situations a Hypergamma or Cinegamma works well because below about 70% exposure Hypergammas and cinegammas are very similar to Rec-709 so you will have lots of data in the shadows and mid range where you really need it. The highlights will be up in the highlight roll off area where the data levels or number of recorded shades are rolled off. So the highlights still get recorded, perhaps without clipping, but you are only giving away a small amount of data to do this. The highlights possibly won’t look quite as nice as if recorded with log, but they are typically only a small part of the scene and the rest of the scene especially the shadows and mid tones will end up looking much better as the noise will be smoother and there will be more data in that all important mid-range.

 

Treat it like a film camera!

If you have a modern camera that can record log or raw and has 13 stops or more of dynamic range you need to stop thinking “video” and think “film”.

A big mistake most traditional video camera operators make with these big DR cameras is to treat them as they would a typical limited dynamic range video camera and constantly worry and obsess about protecting highlights. Why do we do this? Well probably because that’s what you do with cameras with a very limited range and that’s probably what you have had drummed into you for years. But now with modern large sensor cameras everything changes. When you get to a 14 stop range camera, even if you choose to shoot 2 stops over exposed  (perhaps by using 500 EI on an FS7 or F5) you still have as much or more over exposure range as a conventional video camera and the highlight range that you do have is not subject to a knee or other similar acute highlight compression. So any highlights will contain a ton of high quality, usable picture information. By shooting over exposed by a controlled amount (1 to 2 stops), perhaps by using a low EI you gain very big improvements in the signal to noise ratio and get better saturated colors (opening the aperture lets more light onto the sensor, your colors will be better recorded). This allows you to pull a lot more information out of the data thin shadows and mid range. Most cameras that use log have very little data in the shadows. If you are recording with a 10 bit codec cameras that use variations of the Cineon log curve (Arri LogC, Sony S-Log3, Panasonic V-Log) only have about 80 luma shades covering the first 4 stops of exposure in total. Above the 4th stop the amount of data per stop increases rapidly so a little bit of deliberate over exposure really helps lift your darkest shadows up out of the noise and mire. Up in the highlights each stop has exactly the same amount of data, so over exposing a bit doesn’t compress the highlights as it would with a conventional camera, so a bit of mild over exposure is normally not noticeable.

Really with a 14 stop log camera you want to treat it like film, not video. Just like film, a 14 stop log camera will almost always benefit from a controlled amount of over exposure, highlights will rarely suffer or look bad just because you’re one stop hot, but he shadows and midtones will be significantly improved. And just like film, if you under expose log you will take a big hit. You will loose a lot of shadow information very quickly, have less color, it will be noisy and the highlight benefit will be marginal.

More info on CMOS sensor grid artefacts.

Cameras with bayer CMOS sensors can in certain circumstances suffer from an image artefact that appears as a grid pattern across the image. The actual artefact is normally the result of red and blue pixels that are brighter than they should be which gives a magenta type flare effect. However sometimes re-scaling an image containing this artefact can result in what looks like a grid type pattern as some pixels may be dropped or added together during the re scaling and this makes the artefact show up as a grip superimposed over the image.

grid-pattern-1024x576 More info on CMOS sensor grid artefacts.
Grid type artefact.

The cause of this artefact is most likely off-axis light somehow falling on the sensor. This off axis light could come from an internal reflection within the camera or the lens. It’s known that with the F5/F55 and FS7 cameras that a very strong light source that is just out of shot, just above or below the image frame can in some circumstances with some lenses result in this artefact. But this problem can occur with almost any CMOS Bayer camera, it’s not just a Sony problem.

The cure is actually very simple, use a flag or lens hood to prevent off axis light from entering the lens. This is best practice anyway.

So what’s going on, why does it happen?sony-grid-artefact-explained More info on CMOS sensor grid artefacts.

When white light falls on a bayer sensor it passes through color filters before hitting the pixel that measures the light level. The color filters are slightly above the pixels. For white light the amount of light that passes through each color filter is different.  I don’t know the actual ratios of the different colors, it will vary from sensor to sensor, but green is the predominant color with red and blue being considerably lower, I’ve used some made up values to illustrate what is going on, these are not the true values, but should illustrate the point.

In the illustration above when the blue pixel see’s 10%, green see 70% and red 20%, after processing the output would be white. If the light falling on the sensor is on axis, ie coming directly, straight through the lens then everything is fine.

But if somehow the light falls on the sensor off axis at an oblique angle then it is possible that the light that passes through the blue filter may fall on the green pixel, or the light from the green filter may fall on the red pixel etc. So instead of nice white light the sensor pixels would think they are seeing light with an unusually high red and blue component. If you viewed the image pixel for pixel it would have very bright red pixels, bright blue pixels and dark green pixels. When combined together instead of white you would get Pink or Blue. This is the kind of pattern that can result in the grid type artefact seen on many CMOS bayer sensors when there are problems with off axis light.

This is a very rare problem and only occurs in certain circumstances. But when it does occur it can spoil an otherwise good shot. It happens more with full frame lenses than with lenses designed for super 35mm or APSC and wide angles tend to be the biggest offenders as their wide Field of View  (FoV) allows light to enter the optical path at acute angles. It’s a problem with DSLR lenses designed for large 4:3 shaped sensors rather than the various wide screen format that we shoot video in today. All that extra light above and below the desired widescreen frame, if it isn’t prevented from entering the lens has to go somewhere. Unfortunately once it enters the cameras optical path it can be reflected off things like the very edge of the optical low pass filter, the ND filters or the face of the sensor itself.

The cure is very simple and should be standard practice anyway. Use a sun shade, matte box or other flag to prevent light from out of the frame entering the lens. This will prevent this problem from happening and it will also reduce flare and maximise contrast. Those expensive matte boxes that we all like to dress up our cameras with really can help when used and adjusted correctly.

I have found that adding a simple mask in front of the lens or using a matte box such as any of the Vocas matte boxes with eyebrows will eliminate the issue. Many matte boxes will have the ability to be fitted with a 16:9 or 2.40:1 mask ( also know as Mattes hence the name Matte Box) ahead of the filter trays. It’s one of the key reason why Matte Boxes were developed.

IMG_1022 More info on CMOS sensor grid artefacts.
Note the clamp inside the hood for holding a mask in front of the filters on this Vocas MB216 Matte Box. Not also how the Matte Box’s aperture is 16:9 rather than square to help cut out of frame light.
SMB-1_mpa_04-1024x576 More info on CMOS sensor grid artefacts.
Arri Matte Box with Matte selection.

You should also try to make sure the size of the matte box you use is appropriate to the FOV of the lenses that you are using. An excessively large Matte Box isn’t going to cut as much light as a correctly sized one.  I made a number of screw on masks for my lenses by taking a clear glass or UV filter and adding a couple of strips of black electrical tape to the rear of the filter to produce a mask for the top and bottom of the lens. With zoom lenses if you make this mask such that it can’t be seen in the shot at the wide end the mask is effective throughout the entire zoom range.

f5-f55-mask More info on CMOS sensor grid artefacts.

Many cinema lenses include a mask for 17:9 or a similar wide screen aperture inside the lens.

 

Tales of exposure from the grading suite.

I had the pleasure of listening to Pablo Garcia Soriano the resident DiT/Colorist at the Sony Digital Motion Picture Center at Pinewood Studios last week talk about grading modern digital cinema video cameras during the WTS event .

The thrust of his talk was about exposure and how getting the exposure right during the shoot makes a huge difference in how much you can grade the footage in post. His main observation was that many people are under exposing the camera and this leads to excessive noise which makes the pictures hard to grade.

There isn’t really any real way to reduce the noise in a video camera because nothing you normally do can change the sensitivity of the sensor or the amount of noise it produces. Sure, noise reduction can mask noise, but it doesn’t really get rid of it and it often introduces other artefacts. So the only way to change the all important signal to noise ratio, if you can’t change the noise, is to change the signal.

In a video camera that means opening the aperture and letting in more light. More light means a bigger video signal and as the noise remains more or less constant that means a better signal to noise ratio.

If you are shooting log or raw then you do have a fair amount of leeway with your exposure. You can’t go completely crazy with log, but you can often over expose by a stop or two with no major issues. You know, I really don’t like using the term “over-expose”  in these situations. But that’s what you might want to do, to let in up to 2 stops more light than you would normally.

In photography, photographers shooting raw have been using a technique called exposing to the right (ETTR) for a long time. The term comes from the use of a histogram to gauge exposure and then exposing so the the signal goes as far to the right on the histogram as possible (the right being the “bright” side of the scale). If you really wanted to have the best possible signal to noise ratio you could use this method for video too. But ETTR means setting your exposure based on your brightest highlights and as highlights will be different from shot to shot this means the mid range of you shot will go up and down in exposure depending on how bright the highlights are. This is a nightmare for the colorist as it’s the mid-tones and mid range that is the most important, this is what the viewer notices more than anything else. If these are all over the place the colorist has to work very hard to normalise the levels and it can lead to a lot of variability in the footage.  So while ETTR might be the best way to get the very best signal to noise ratio (SNR), you still need to be consistent from shot to shot so really you need to expose for mid range consistency, but shift that mid range a little brighter to get a better SNR.

Pablo told his audience that just about any modern digital cinema camera will happily tolerate at least 3/4 of a stop of over exposure and he would always prefer footage with very slightly clipped highlights rather than deep shadows lost in the noise. He showed a lovely example of a dark red car that was “correctly” exposed. The deep red body panels of the car were full of noise and this made grading the shot really tough even though it had been exposed by the book.

When I shoot with my F5 or FS7 I always rate them a stop slower that the native ISO of 2000. So I set my EI to 1000 or even 800 and this gives me great results. With the F55 I rate that at 800 or even 640EI. The F65 at 400EI.

If you ever get offered a chance to see one of Pablo’s demos at the DMPCE go and have a listen. He’s very good.

Exposing and Using S-Log2 on the Sony A7s (or any other Alpha camera). Part One: Gamma and Exposure.

This document has been prepared independently of Sony. It is based on my own findings having used the camera and tested various exposure levels and methods. Part 2 which explains how to use LUT’s to correct the footage in the edit suite or post production is here: http://www.xdcam-user.com/2014/10/using-s-log2-from-the-a7s-in-post-production/

If you find this useful please consider buying me a coffee or a beer. I’m not paid to write these articles.


Type



pixel Exposing and Using S-Log2 on the Sony A7s (or any other Alpha camera). Part One: Gamma and Exposure.

One of the really nice features of the Sony A7s and Sony’s other Alpha cameras, including the A6300, A6500 etc is the ability to use different gamma curves and in particular the Sony S-Log2 gamma curve.

What are gamma curves?

All conventional cameras use gamma curves. The gamma curve is there to make the images captured easier to manage by making the file size smaller than it would be without a gamma curve. When TV was first developed the gamma curve in the camera made the signal small enough to be broadcast by a transmitter and then the gamma curve in the TV set (which is the inverse of the one in the camera) expanded the signal back to a normal viewing range. The current standard for broadcast TV is called “Recommendation BT-709”, often shortened to Rec-709. This gamma curve is based on standards developed over 60 years ago and camera technology has advanced a lot since then! Even so, almost every TV and monitor made today is made to the Rec-709 standard or something very similar. Many modern cameras can capture a brightness range, also known as dynamic range, that far exceed the Rec-709 standard.

The limitations of standard gammas.

As gamma effects the dark to light range of the image, it also effects the contrast of the image. Normal television gamma has a limited dynamic range (about 6 to 7 stops) and as a result also has a limited contrast range.

Normal-Gamma Exposing and Using S-Log2 on the Sony A7s (or any other Alpha camera). Part One: Gamma and Exposure.
When shooting a high contrast scene with conventional gamma the brightest highlights and the darkest shadows cannot be recorded. The contrast on the TV or monitor will however be correct as the camera captures the same contrast range as the monitor is able to display.

Normally the gamma curve used in the camera is designed to match the gamma curve used by the TV or monitor. This way the contrast range of the camera and the contrast range of the display will be matched. So the contrast on the TV screen will match the contrast of the scene being filmed and the picture will look “normal”. However the limited dynamic range may mean that very bright or very dark objects cannot be accurately reproduced as these may exceed the gammas dynamic range.

Slide5 Exposing and Using S-Log2 on the Sony A7s (or any other Alpha camera). Part One: Gamma and Exposure.
Although the dynamic range of Rec-709 may not always capture the entire range of the scene being shot, as the gamma of the camera matches the gamma of the TV the contrast will appear correct.

The over exposure typical of a restricted range gamma such as Rec-709  is commonly seen as bright clouds in the sky becoming white over exposed blobs or bright areas on faces becoming areas of flat white. Objects in shade or shadow areas of the scene are simply too dark to be seen. But between the overexposed areas and any under exposure the contrast looks natural and true to life.

Slog-709-FH Exposing and Using S-Log2 on the Sony A7s (or any other Alpha camera). Part One: Gamma and Exposure.
Typical limited Rec-709 exposure range. Contrast is good but the clouds are over exposed and look un-natural.

Log Gamma.

Log gamma, such as Sony’s S-Log2, allows the camera to capture a much greater brightness range or dynamic range than is possible when shooting with conventional television gamma. Dynamic range is the range from light to dark that the camera can capture or the range that the monitor or TV can display within one image. It is the range from the deepest blacks to the brightest whites that can be captured or shown at the same time.

There are some things that need to be considered before you get too excited about the possibility of capturing this much greater dynamic range. The primary one being that if the camera is set to S-log2 and the TV or monitor is a normal Rec-709 TV (as most are) then there is no way the TV can correctly display the image being captured, the TV just doesn’t have the range to show everything that the camera with it’s high range log gamma can capture accurately.

Fixed Recording Range For Both Standard and Log Gamma.

The signal range and signal levels used to record a video signal are normally described in percent. Where black is 0% and the brightest thing that can be recorded is normally recorded at 100% to 109%. Most modern video cameras actually record the brightest objects at 109%. The important thing to remember though is that the recording range is fixed. Even when you change gamma curve the camera is still constrained by the zero to 109% recording range. The recording range does not change whether you are recording Rec-709 or S-log2. So log gamma’s like S-Log2 must squeeze a much bigger signal range into the same recording range as used by conventional Rec-709 recordings.

Slide6 Exposing and Using S-Log2 on the Sony A7s (or any other Alpha camera). Part One: Gamma and Exposure.
Log gamma squeezes the scenes large range to fit in the camera’s normal 0%-109% recording range.

Recording S-Log2.

In order to record using S-log2 with the A7s you need to use a picture profile. The picture profiles give you several recording gamma options. For S-log2 you should use Picture Profile 7 which is already set up for S-log2 and S-Gamut by default (for information on gamuts see this article). In addition you should ALWAYS use the cameras native ISO which is 3200 ISO and it is normally preferable to use a preset white balance. Using any other ISO with S-log2 will not allow you to get the full benefit of the full 14 stops of dynamic range that S-log2 can deliver. In most of the Alpha cameras you now also have the ability to use a different version of S-log, – S-Log3 and this is found in picture profiles 8 and 9. You can use S-Log3 if you wish, but S-Log2 was designed from the outset by Sony to work with digital camera sensors. S-Log3 is based on an older curve designed for film transfers to a 10 bit recording. As a result when using a camera that only has 8 bit recording with a limited number of code values, S-Log2 tends to be more efficient and yield a better end result. This is what it was designed for.

Grey Cards and White Cards.

Before I go further let me introduce you to grey and white cards in case you have not come across them before. Don’t panic you don’t have to own one, although I would recommend getting a grey card such as the Lastolite EzyBalance if you don’t have one. But it is useful to understand what they are.

The 90% White Card.

The 90% white card is a card or chart that reflects 90% of the light falling on it. This will be a card that looks very similar in brightness to a piece of ordinary white paper, it should be pure white, some printer papers are bleached or coloured very slightly blue to make them appear “brilliant white”  (as you will see later in many cases it is possible to use an ordinary piece of white paper in place of a 90% white card for exposure).

The Grey Card.

The 18% grey card, also often called “middle grey” card, is a card that reflects 18% of the light falling on it. Obviously it will appear much darker than the white card. Visually to us humans an 18% grey card appears to be half way between white and black, hence it’s other name, “middle grey”.

Middle grey is important because the average brightness level of most typical scenes tends to be around the middle grey brightness value. Another key thing about middle grey is that because it falls in the middle of our exposure range it makes it a very handy reference level when measuring exposure as it is less likely to be effected by highlight compression than a 90% white card.

Exposing White and Middle Grey.

Coming back to Rec-709 and conventional TV’s and monitors. If we want a piece of white paper to look bright and white on a TV we would record it and then show it at somewhere around 85% to 95% of the screens full brightness range. This doesn’t leave much room for things brighter than a white piece of paper! Things like clouds in the sky, a shiny car, a bright window or a direct light source such as a lamp or other light.  In order to make it possible for S-log2 to record a much greater dynamic range the recording level for white and mid tones is shifted down. Instead of recording white at 85%-95%, when using S-log2 or S-Log3 it is recommended by Sony that white is recorded at around 60%. For S-Log2 Middle grey moves down too, instead of being recorded at 42%-43% (the normal level for Rec-709) it’s recorded at just 32% with S-Log2 (S-log3 uses 41%).

By recording everything white (ie a white piece of paper) and darker in a  lower range, we free up  lot of extra space above the white recording level, within the full recording range, to record all those bright highlights in any scene that would be impossible to record with conventional gammas where there is only 10% to 20% from white at 90% to the peak of the recording range at 100 to 109%.

Slide7 Exposing and Using S-Log2 on the Sony A7s (or any other Alpha camera). Part One: Gamma and Exposure.
To make room for the extra dynamic range and the ability to record very bright objects, white and mid tones are shifted down in level by the S-log2 gamma curve. As a result, white, mid tones etc will be displayed darker than normally expected with conventional gamma.

As S-Log2 and S-Log3 normally shift a lot of the recording levels downwards, if we show a scene shot with S-Log2 or S-log3 that has been exposed correctly on a conventional TV or monitor it will look dark due to the lower recording levels. In addition it will look flat with very low contrast as we are now squeezing a much bigger dynamic range into the limited conventional Rec-709 display range of a normal TV or computer monitor.

Slide8 Exposing and Using S-Log2 on the Sony A7s (or any other Alpha camera). Part One: Gamma and Exposure.
The on screen contrast appears reduced as the capture contrast is greater than the display contrast.

This on screen reduction in contrast and the darker levels are actually perfectly normal when shooting using log gamma, this is how it is supposed to look on a normal monitor or TV. So don’t be alarmed if when shooting using S-Log your images look a little darker and flatter than perhaps you are used to when shooting with a standard gamma. You will adjust the S-Log footage in post production to restore the brightness and contrast later.

S-log2-correct-FH-1024x576 Exposing and Using S-Log2 on the Sony A7s (or any other Alpha camera). Part One: Gamma and Exposure.
Correctly exposed S-Log2 can look dark and washed out.

The post production adjustment of S-Log2 and S-log3 is very important and one of the keys to getting the very best finished images. The S-Log recording acts as a digital negative and by “processing” this digital negative in post production (normally referred to as “grading”) we manipulate the large 14 stop dynamic range of the captured image to fit within the limited display range of a Rec-709 TV in a pleasing manner. This may mean pulling up the mid range a bit, pulling down the highlights and bit and generally shifting the brightness and colour levels of different parts of the image around  (see PART 2 for more post production information).

SLog-2 and 10 bit or 8 bit data.

Originally Slog-2 was designed for use on high end digital cinema cameras such as Sony’s F65 camera. These cameras have the ability to record using 10 bit data. A 10 bit recording can have up to around 1000 shades of grey from black to white. The A7s however uses 8 bit recording which only has a maximum of 235 shades from black to white. Normally 8 bit recording is perfectly OK as most transmission and display standards are also 8 bit. Shoot with an 8 bit camera and then display that image directly via an 8 bit system and nothing is lost. However when you start to grade and manipulate the image the difference between 8 bit and 10 bit becomes more significant. If you start to shift levels around, perhaps stretching out some parts of the image then the increased tonal resolution of a 10 bit recording helps maintain the very highest image quality. Photographers that have shot using both jpeg and raw will know how much more flexibility the 12 bit (or more) raw files have compared to the 8 bit jpeg’s. However they will also know that 8 bit jpeg’s can be also adjusted, provided you don’t need to make very large adjustments.

Contrary to popular belief heavy grading of 8 bit footage does not necessarily lead to banding in footage across smooth surfaces except in extreme cases. Banding is more commonly a result of compression artefacts such as macro blocking. This is especially common with very highly compressed codecs such as AVCHD. The 50Mbps XAVC-S codec used in the Sony Alpha cameras is a very good codec, far superior to AVCHD and as a result compression artefacts are significantly reduced, so banding will be less of an issue than with other lower quality codecs. If you’re going to shoot using S-Log2, some grading will be necessary and as we only have 8 bit recordings we must take care to expose our material in such a way as to minimise how far we will need to push and pull the material.

Getting Your Exposure Right.

When S-Log2 was developed the engineers at Sony produced tables that specified the correct exposure levels for s-Log2 which are:

exposure-table1 Exposing and Using S-Log2 on the Sony A7s (or any other Alpha camera). Part One: Gamma and Exposure.As you can see the nominal “correct” exposure for S-Log2 is a lot lower than the levels used for display on a typical Rec-709 TV or monitor. This is why correctly exposed s-log2 looks dark on a conventional TV. The implication of this is that when you grade your footage in post production you will have to shift the S-log2 levels up quite a long way. This may not be ideal with an 8 bit codec, so I decided to carefully test this to determine the optimum exposure level for the A7s.

Correct Exposure.

The panel of images below is from the A7s recording S-log2 and exposed at the Sony recommended “correct” 32% middle grey level. The correct exposure was determined using a grey card and an external waveform monitor connected to the cameras HDMI output. Then the S-log2 was corrected in post production to normal Rec-709 levels using a Look Up Table (LUT – more on LUT’s in part 2). You can also see the viewfinder display from the camera. If you click on the image below you can expand it to full size. Sorry about the shadow from the laundry line, I didn’t see this when I was shooting the test shots!

Slog2-correct-exposure-panel-1024x564 Exposing and Using S-Log2 on the Sony A7s (or any other Alpha camera). Part One: Gamma and Exposure.
Correctly exposed S-Log2 from A7s.

From this you can see just how dark and low contrast looking the original correctly exposed S-log2 is and how much more vibrant the corrected Rec-709 image is. I have also indicated where on the cameras histogram middle grey and white are. Note how much space there is to the right of white on the histogram. This is where the extra highlight or over exposure range of S-log2 can be recorded. When correctly exposed S-log2 has an exposure range of 6 stops above middle grey and 8 stops under.

Over Exposing or “Pushing” S-log2.

If we deliberately raise the exposure level above the Sony recommended levels (known as pushing the exposure), assuming you grade the image to the same final levels some interesting things happen.

For each stop we raise the exposure level you will have 1 stop (which is the same as 6db) less noise. So the final images will have half as much noise for each stop up you go. This is a result of exposing the image brighter and as a result not needing to raise the levels in post as far as you would if exposed at the normal level.

You will loose one stop of over exposure headroom, but gain one stop of under exposure headroom.

Bright highlights will be moved upwards into the most compressed part of the log gamma curve. This can result in a loss of texture in highlights.

Skin tones and mid tones move closer to normal Rec-709 levels, so less manipulation is need for this part of the image in post production.

This last point is important for the A7s with it’s 8 bit codec, so this is the area I looked at most closely. What happens to skin tones and textures when we raise the exposure?

Exposing at +1, +2 and +3 Stops.

Below are another 3 panels from the A7s, shot at +1 stop, +2 stops and +3 stops. Again you can click on the images if you wish to view them full size.

Slog2-plus1-exposure-panel-1024x564 Exposing and Using S-Log2 on the Sony A7s (or any other Alpha camera). Part One: Gamma and Exposure.
A7s S-Log2 over exposed by one stop.
Slog2-plus2-exposure-panel-1024x564 Exposing and Using S-Log2 on the Sony A7s (or any other Alpha camera). Part One: Gamma and Exposure.
A7s S-Log2 over exposed by 2 stops.
Slog2-plus3-exposure-panel-1024x564 Exposing and Using S-Log2 on the Sony A7s (or any other Alpha camera). Part One: Gamma and Exposure.
A7s S-Log2 over exposed by 3 stops.

Looking at these results closely you can see that when you increase the exposure by 1 stop over the Sony specified correct level for S-log2 there is a very useful reduction in noise, not that the A7s is particularly noisy to start with, but you do get a noticeably cleaner image.

Below are 4 crops from the same images, after grading. I really recommend you view these images full size on a good quality monitor. Click on the image to view larger or full size.

A7s-over-exposure Exposing and Using S-Log2 on the Sony A7s (or any other Alpha camera). Part One: Gamma and Exposure.
Crops at different exposure of LUT corrected A7s S-log2 footage.

The noise reduction at higher exposures compared to the base exposure is very clear to see if you look at the black edge of the colour checker chart (the coloured squares), although the difference between +2 and +3 stops is very small. You can also see further into the shadows in the +3 stop image compared to the base exposure. A more subtle but important effect is that as the exposure goes up the visible texture of the wooden clothes peg decreases. The grain can be clearly seen at the base level but by +3 stops it has vanished. This is caused by the highlights creeping into the more compressed part of the log gamma curve. The same thing is happening to the skin tones in the +3 stop image, there is some reduction of the most subtle textures.

From this we can see that for mid tones and skin tones you can afford to expose between 1 and 2 stops above the Sony recommended base level. More than 2 stops over and brighter skin tones and any other brighter textures start to be lost. The noise reduction gain by shooting between one and 2 stops over is certainly beneficial. The down side to this though is that we are reducing the over amount of exposure headroom.

Slide02 Exposing and Using S-Log2 on the Sony A7s (or any other Alpha camera). Part One: Gamma and Exposure.
As you raise the exposure level you reduce the over exposure headroom.

Given everything I have seen with this 8 bit and almost every other 8 bit camera my recommendation is to shoot between the Sony recommended base S-log2 level and up to two stops over this level. I would try to avoid shooting more than 2 stops over as this is where you will start to see some loss of texture in brighter skin tones and brighter textures.  Exactly where you set your exposure will depend on the highlights in the scene. If you are shooting a very bright scene you will possibly need to shoot at the Sony recommended level to get the very best over exposure headroom. If you are able to expose higher without significantly compromising any highlights then you should aim to be up to 2 stops over base. But whatever you do never expose darker than the Sony base level, this will normally look really nasty.

Determining The Correct Exposure.

The challenge of course is determining where your exposure actually is. Fortunately as we have seen, provided you in the right ball park, S-log2 is quite forgiving, so if you are a little bit over exposed it’s probably not going to hurt your images much. If you have a waveform monitor then you can use that to set your exposure according to the table below. If you don’t have proper white or grey cards you can use a piece of normal white paper. Although slightly less accurate this will get you very close to where you want to be. Do note that white paper tends to be a little brighter than a dedicated 90% reflectivity white card. If you don’t have any white paper then you can use skin tones, again a bit less accurate but you should end up in the right zone.

A7s-exposure-levels-1024x358 Exposing and Using S-Log2 on the Sony A7s (or any other Alpha camera). Part One: Gamma and Exposure.
My suggested exposure levels for the Sony A7s. The “sweet spot” is from normal to +2 over.

If you don’t have an external waveform monitor then you do still have some good options. Sadly although the camera does have zebras, these are not terribly useful for S-log2 as the lowest the zebras can go is 70%.

Light Meter: You could use a conventional photography light meter. If you do choose to use a light meter I would recommend checking the calibration of the light meter against the camera first.

Mark 1 Eyeball: You could simply eyeball the exposure looking at the viewfinder or rear screen but this is tricky when the image is very flat.

In Camera Metering: The cameras built in metering system, like the majority of DSLR’s is calibrated for middle grey. By default the camera uses multi-point metering to measure the average brightness of several points across the scene to determine the scenes average brightness and from there set the correct base S-log2 exposure.

Auto Exposure:

When you are using S-Log2, auto exposure in most cases will be very close to the correct base exposure if you use the default Multi-Zone exposure metering. The camera will take an average exposure reading for the scene and automatically adjust the exposure to the Sony recommended 32% middle grey exposure level based on this average. In the P, A and S modes you can then use the exposure compensation dial to offset the exposure should you wish. My recommendation would be to add +1 or +2 stops via the dial. Then observe the histogram to ensure that you don’t have any significant over exposure. If you do then reduce the exposure compensation. Lots of peaks to the far right of the histogram is an indication of over exposure.

Manual Exposure And Internal Metering.

If you are exposing manually you will see a small M.M. indication at the bottom of the LCD display with a +/- number. In the eyepiece viewfinder this appears as a scale that runs from -5 to +5, in S-log2 only the -2 to +2 part of the scale is used. In both cases this is how far the camera thinks you are away from the optimum exposure. + meaning the camera is over exposed, – meaning under.

A7s-VF-MM Exposing and Using S-Log2 on the Sony A7s (or any other Alpha camera). Part One: Gamma and Exposure.
A7s Viewfinder indications in manual exposure mode showing both M.M. offset from metered exposure and histogram.

In the image above we can see the M.M. indication is +0.3, in the eyepiece you would see a small arrow one bar to the right of “0” , indicating the cameras multi zone metering thinks the shot is just a little over exposed, even though the shot has been carefully exposed using a grey card and external waveform monitor. This error is probably due to the large amount of white in the shot, white shirt, white card, test charts with a lot of brighter than grey shades.  In practice an error of 0.3 of a stop is not going to cause any real issues, so even if this was exposed by setting  the exposure so that you have “M.M. 0.0” the exposure would be accurate enough. But it shows that multi point exposure averaging is easily confused.

The scene above is a fairly normal scene, not excessively bright, not particularly dark. If shooting a snow scene for example the cameras multi point averaging would almost certainly result in an under exposed shot as the camera attempts to bring the bright snow in the scene down to the average middle grey level. If shooting a well lit face against a very dark background then the averaging might try to bring the background up and the shot may end up overexposed.

If you want really accurate exposure then you should put the cameras metering system into the spot metering mode where instead of taking an average of various points across the scene the camera will just measure the exposure at the very center of the image.

spot-metering-mode Exposing and Using S-Log2 on the Sony A7s (or any other Alpha camera). Part One: Gamma and Exposure.
A7s Spot Metering Mode.

You can then use a grey card to very accurately set the exposure. Simply place the circular shaped symbol at the center of the viewfinder display over a grey card and set the exposure so that M.M is 0.0 for the correct S-Log2 base exposure. To expose 1 stop over with a grey card, set M.M. +1.0 and two stops over M.M. +2.0 (not flashing, flashing indicates more than +2 stops).

MM-00-grey Exposing and Using S-Log2 on the Sony A7s (or any other Alpha camera). Part One: Gamma and Exposure.
Using Spot Metering to set exposure correctly for S-log2. MM 0.0.

One small issue with this is that the camera will only display a M.M. range of -2.0 to +2.0 stops. Provided you don’t want to go more than 2 stops over base then you will be fine with a grey card.

Using White Instead of Grey:

If you don’t have a grey card then you can use a 90% reflectivity white target. As white is 2 stops brighter than middle grey when S-Log2 is correctly exposed the 90% white should indicate M.M +2.0.

MM-plus2-white Exposing and Using S-Log2 on the Sony A7s (or any other Alpha camera). Part One: Gamma and Exposure.
Using spot metering to set the correct exposure for S-Log2. M.M should read M.M +2.0 for a 90% reflectivity white target.

Once you have established the correct exposure you can then open the iris by 1 or two stops to increase the exposure. Or halve the shutter speed to gain a one stop brighter exposure. Each time you halve the shutter speed your exposure becomes one stop brighter, so divide the shutter speed by 4 to gain a 2 stop increase in exposure. As always you should observe the histogram to check for any over exposure. White peaks at the far right of the histogram or disappearing completely off the right of the histogram is an indication of over-exposure. In this case reduce your exposure back down towards the base exposure level (M.M 0.0 with a a grey card).

Exposure Summary:

I recommend using an exposure between the “correct” base S-Log2 exposure level of middle grey at 32% and two stops over this. I would not recommend going more than 2 stops over over base.

In the P, A and S auto exposure modes, when using the default multi-zone metering the camera will set the base S-log2 exposure based on the average scene brightness. For most typical scenes this average should be very close to middle grey. This exposure can then be increased (brightened) by up to 2 stops using the exposure compensation dial.

In manual exposure the “M.M.” number displayed at the bottom of the viewfinder display is how far you are from the correct base S-log2 exposure. M.M. +2.0 indicates +2 stops over base. If using multi zone metering (the cameras default) this exposure will be based on the scenes average brightness.

If you set the metering to “Spot” you can use a grey card centred in the image to determine the correct base exposure and up to 2 stops of over exposure via the M.M. indication when shooting manually.

In Part 2:

In part two I will take a look at grading the S-log2 from the A7s and how to get the very best from the S-log2 images by using Look Up Tables (LUT’s).

PART 2 – How to handle the footage in post production is here.

NORTHERN LIGHTS Tours and workshops.

Don’t forget I run storm chasing and Northern Lights expeditions every year.  These are amazing expeditions by snowmobile up on to the Finnmarksvidda. We go ice fishing, dog sledding, exploring, cook a meal in a tent and enjoy traditional Norwegian saunas.

More information here.

sky-full-of-Aurora-1024x683 Exposing and Using S-Log2 on the Sony A7s (or any other Alpha camera). Part One: Gamma and Exposure.
Northern Lights over our cabins in Norway.

Quick tips for shooting lightning – Video and Stills.

day2-frame-2-300x155 Quick tips for shooting lightning - Video and Stills.
At night we shoot lightning!

With the UK set to see a couple of days of strong and severe thunderstorms I thought I would put together a very quick guide to shooting lightning with both stills cameras and video cameras. Your first issue will be finding somewhere dry to shoot from, you don’t want rain on your camera or lens. You also do need to consider safety. Lightning is dangerous, it can strike many miles from a thunderstorm. If you can hear thunder you are in the strike risk area, so do take care. One of the safest places to be in a thunderstorm is inside a car. If the car is struck the electricity will pass through the body of the car and not through the occupants, before jumping from the underside of the car to the ground. If you are shooting from a car stay inside the car, don’t sit with your feet out of the door or any part of you touching the ground. Don’t sit in the car while holding on to a camera on a tripod outside the car. Don’t stand under trees, they can explode when struck by lightning, don’t stand on the very top of a hill. Use your common sense.

day2-frame3-300x168 Quick tips for shooting lightning - Video and Stills.
Long exposure captures great nigh time lightning.

For either stills or video you’re really going to want to use a tripod to get the very best results. As you often get strong winds around thunderstorms you want a good stable tripod. If it is windy keep a close eye on the camera and tripod, you don’t want it blown over by a strong gust of wind.

A wide angle lens will increase your chances of getting a lightning bolt in your shot, but the wider the shot the less detail you will see in the lightning bolt. You can always crop in to a wide shot a bit if it’s too wide. I like to have something in the foreground to give some interest to the image, but try to avoid too many obstructions to the skyline as these will block your view of the lightning.

NIGHT FOCUS:

This is probably the easiest for still photos, but it has many challenges. One is focus as it’s hard to focus on a brief flash of lightning. You will need to use manual focus, autofocus will not work. Start by focussing on a very distant object, perhaps lights on the horizon, the moon, stars or any other VERY distant object, preferably a mile or more away. Then check and double check your focus. Lightning is very fine and if it’s out of focus it will ruin the shot. If you don’t have anything to focus on set the lens to infinity, the sideways “8” symbol is infinity and there will normally be a line to mark the point of infinity focus. Infinity is often NOT at the very end of the lenses focus travel so check for the proper infinity mark. By the way, take a torch/flashlight if your going out in the dark!

STILL PHOTO’s or DSLR AT NIGHT:

You will need to use a tripod. If you have a cable release or other electronic shutter release use it to trigger the camera to prevent shaking the camera as you will need to use a long exposure. As you will be using a long exposure you want to use a low ISO. I typically use 200ISO with an exposure of between 10 and 30 seconds depending on the frequency of the lightning and how bright the surrounding area is. If you are in a town or city with lots of street light you will probably need to use a shorter exposure, maybe 10 to 15 seconds. Out in the countryside you might be able to use 20 to 30 seconds. For the aperture you don’t want super shallow depth of field as this will show up any focus errors, so don’t use your lens wide open. I normally use somewhere around f4 to f8, so f5.6 is probably a good starting point. Take some test shots and check that you are not over exposed.

As a starting point try: 200ISO, f5.6, 10 second exposures, manual focus.

Once the camera is set, it simply a case of snapping away taking pictures until you get lucky and capture one in the frame. It takes a bit of luck and patience, but don’t give up too soon, just keep snapping away. You can just delete all the no good shots later.

700-lightning-sunset-close-1024x576 Quick tips for shooting lightning - Video and Stills.
Evening thunderstorm in Tucson, Arizona

DAY and NIGHT VIDEO:

If your camcorder has a CMOS sensor (as most do these days) you want to use the slowest shutter speed that you can get away with. If you can control the shutter manually turn it off or reduce it to 1/25 or 1/30. This will reduce the likelihood of you getting lightning bolts that only go half way down the screen, an effect know as “rolling shutter” or “flash band”. If shooting after dark, if you have a camera with full manual control then instead of shooting at the usual 24, 25 or 30 frames per second, consider shooting at half of this, perhaps at 12, 12.5 or 15 frames per second (S&Q motion, slow shutter etc), again with the shutter set to OFF. While this does mean that the motion in your final video will be sped up it almost guarantees that you won’t get any rolling shutter issues. You will need to have the camera on a tripod if doing this to prevent excessive image blur from movement of the camera. The slightly sped up video can also give the pleasing (but fake) impression that the lightning is more frequent than it really is making your shots more dramtic. If you don’t want this simply play the video back at half speed.

STILL PHOTOS DURING THE DAY:

This is really tough unless you have special equipment. You can’t use a long exposure as you would at night because the bright daytime light will wash out the lightning bolts.

Very often a lightning bolt is made up of several flashes in rapid succession. If you do have fast enough reactions and a fast enough camera, you can get the secondary flashes. You will need to use manual focus and manual exposure so there isn’t a delay while the  camera thinks about focus and exposure which delays the release of the shutter. Use a tripod with a cable release or remote shutter and use a longish exposure, 1/30th or 1/15th as there can be up to 1/10th of a second delay between flashes and there could be multiple flashes, you don’t want too fast a shutter speed. Set your focus on a very distant object, use a low ISO, again I typically use 100 or 200 ISO. Shoot a couple of test images and set the aperture so that you have a very slightly underexposed shot, may -1EV to -1.5EV, the slightly darker overall image will help the bright lightning show up better. Then it’s just a case of pointing the camera at the storm on a tripod, with your finger on the trigger and try to hit that shutter release as soon as you see any lightning. I find it’s better to not look through the viewfinder, just look in the direction the camera is pointed. You may be lucky, maybe not, a lot will depend on the type of lightning in the storm and your reaction speed. A better way is to use a dedicated lightning trigger such as a Patchmaster: http://www.fotokonijnenberg.nl/patchmaster. This will trigger the camera electronically if it detects any lightning. It’s MUCH faster and can react much quicker than any human, but it still has some lag time so even a lightning trigger won’t capture every bolt.

A final daytime method is to use an adaptation of the night time DSLR method. If you add a strong ND filter a small aperture around f16 and use a low ISO you may be able to get an acceptable long exposure during daytime, perhaps a couple of seconds. Then set the camera to take photo’s continuously (so when you hold the shutter button down the camera will take one photo after another). By locking down a remote shutter release the camera will take a continuous stream of photos with only a very minimal gap between each picture taken. So you have a high likely hood of capturing any lightning bolts, but you will also end up with a lot of pictures that don’t have any lightning in them. You can either discard these empty frames or use all the frames to create a time-lapse video of the storm.

Have fun, stay safe.

If you find the guide useful, please consider buying me a beer or a coffee.


Type



pixel Quick tips for shooting lightning - Video and Stills.

 

LUT’s or LOOK’s when exposing via a LUT on the PMW-F5 or PMW-F55 (and FS7).

First of all. You can use either, LUT’s or Looks. But there is a quite marked difference in the way they behave, especially if you use EI gain.

At the native ISO there is little to choose between them. But just to confirm my earlier suspicions about the way the 3D LOOK’s behave I ran a quick test.

I found that when you lower the EI gain, below native, the output level of the LOOK lowers, so that depending on the EI, the clipping, peak level and middle grey values are different. For example on my PMW-F5 at 500 EI the LC709TypeA LUT has a peak output (clipping) level of just 90% while at 2000 ISO it’s 98%. This also means that middle grey of the LOOK will shift down slightly as you lower the EI. This means that for consistent exposure at different low EI’s you may need to offset your exposure very slightly. It also means that at Native EI if the waveform shows peak levels at 90% you are not overexposed or clipped, but at low EI’s 90% will mean clipped Slog, so beware of this peak level offset.

When you raise the EI of the LOOKS, the input clipping point of the Look profile changes. For each stop of EI you add the LOOK will clip one stop earlier than the underlying Slog. For example set the LC709TypeA LUT to 8000 ISO (on my PMW-F5)  and the LOOK itself hard clips 2 stops before the actual SLog3 clips. So your LOOK will make it appear that your Slog is clipped up to 2 stops before it actually is and the dynamic range and contrast range of the LOOK varies depending on the EI, so again beware.

So, the Looks may give the impression that the Slog is clipped if you use a high ISO and will give the impression that you are not using your full available range at a low ISO. I suspect this is a limitation of 3D LUT tables which only work over a fixed 0 to1 input and output range.

What about the 1D LUT’s? Well the LUT’s don’t cover the full range of the Slog curves so you will never see all of your dynamic range at once. However I feel their behaviour at low and high EI’s is a little bit more intuitive than the level shifts and early clipping of the LOOKs.

The 1D LUT’s will always go to 109%. So there are no middle grey shifts for the LUT, no need to compensate at any ISO. In addition if you see any clipping below 109% then it means your SLog is clipping, for example if you set the camera to 500 ISO (on an F5), when you see the 709(800) LUT clipping at 105% it’s because the Slog is also clipping.

At High ISO’s you won’t see the top end of the SLog’s exposure range anyway because the LUT’s range is less than Slog’s range, but the LUT itself does not clip, instead highlights just go up above 109% and this is in my opinion more intuitive behaviour than the clipped LOOK’s that don’t ever quite reach 100% and clip at lower than 100% even when the Slog itself isn’t clipped.

At the end of the day use the ones that work best for you, just be aware of the limitations of both and that the LUT’s and LOOKs behave very differently. I suggest you test and try both before making any firm decisions.

Personally I prefer to use the 709(800) LUT for exposure as the restricted range matches that of most consumer TV’s etc so I feel this gives me a better idea of how the image may end up looking on a consumers TV. Also I find my Slog exposure more accurate as the LUT’s restricted range means you are more likely to expose within finer limits. In addition as noted above I fell the LUT’s behaviour is more predictable and intuitive at high and low EI’s than the LOOK’s.

In addition the higher contrast makes focus easier. I will often switch in and out of the LUT to look at how the Log is coping with any over exposure. This is my personal preference, but I do also use other LUT’s and Looks in particular the 709TypeA from time to time.

Matte Boxes and Lens Hoods can make your pictures sharper, also a look at the Alphatron Matte Box.

DSC05078-1024x681 Matte Boxes and Lens Hoods can make your pictures sharper, also a look at the Alphatron Matte Box.
Alphatron Matte Box.

So we all like to dress our cameras up with all kinds of accessories. One of the most common being a Matte Box. So, what’s a matte box for? Well the obvious thing is to hold filters for creating an artistic look, for colour correction or light level reduction. But the other very important role is to block unwanted light. I’ll take a brief look at filters later in the article.

We all know that if you shoot into the sun or a bright light source you might get a lens flare in the shot. You know, those sometimes pretty rings of light that can look cool on a good day or ruin a shot on another. But the other thing you can get is lens flare. So whats the difference between “a lens flare” and “lens flare”.

Well, lens flare is when light bounces around inside the lens between the glass elements in an uncontrolled way, some of this unfocussed light making it’s way to the sensor where it spills and bleeds into darker parts of the image reducing contrast and raising the black level. Whenever you reduce the contrast in an image it will appear softer, so to get the sharpest and highest resolution images, we really want to keep as much unwanted light out of the lens as possible. In addition some cameras can suffer from other image artefacts when off-axis light finds it’s way to the edges of the sensor. So anything we can do to stop this happening is obviously a good thing.

Higher end cameras will often have an electronic flare adjustment that pulls down the cameras black level when the overall scene light level gets high. The idea is that this helps compensate for the almost inevitable flare that will occur in the lens when a lot of light enters the lens. This flare setting is normally adjusted on a lens by lens basis as different lenses will flare by different amounts. As lenses get older, very often vapour from the oils and materials used in the construction of the lens will coat the internal glass surfaces with a very fine haze that increases flare. This can make an older lens more prone to flare and is one reason why getting an older but expensive lenses professionally cleaned is often worth the expense. The other thing you can do is to make be sure to use a good matte box or lens shade to prevent excess light from entering the lens.

DSC05083-300x199 Matte Boxes and Lens Hoods can make your pictures sharper, also a look at the Alphatron Matte Box.
A flexible donut or “nun’s Knikers” on the rear of the Matte Box keeps out light from the rear of the Matte Box.

Don’t use a matte box that is excessively large. You want a Matte Box big enough to fit your lenses and hold the size of filters you need. It also needs to be wide enough to allow you to use the aspect ratios you want to shoot in, but no larger. If it’s too big, the shade/hood will be less effective. Make use of an adjustable top flag and side flags to keep out as much light as possible. Looking through the cameras viewfinder bring the flags in close to the lens until they start to creep into the edges of your shot, then back them off just a little bit.

DSC05095-e1397754983842-199x300 Matte Boxes and Lens Hoods can make your pictures sharper, also a look at the Alphatron Matte Box.
A quick release catch on the Alphatron Matte Box allows you to quickly and easily change the donut or nun’s knickers.

Also make sure your rear donut or other light seal is doing it’s job and keeping out the light. A flexible bellows or “nun’s knickers” can be used to allow you to move the matte box forwards so that the lens sits deeper in the nice dark recess of the matte box. Light entering the Matte box from the rear will cause reflections off the back of any filters used, especially any ND filters or glimmer glass filters and this can easily spoil a shot.

A Matte Box can be attached to the lens directly via a clamp ring that clamps around the end of the lens or more commonly attached to rods or bars connected to the bottom of the camera. If you only ever use one lens then a lens clamp might work well for you, but if you swap and change lenses regularly then a rod or rail mount is often easier as a flexible donut will fit a multitude of lenses. The donut on the Alphatron Matte Box will fit a wide range of lenses and the neoprene insert can easily be exchanged or replaced simply by unscrewing the two halves of the donut holder. The neoprene is sandwiched between the two halves and just drops out once released.

DSC05086-300x199 Matte Boxes and Lens Hoods can make your pictures sharper, also a look at the Alphatron Matte Box.
Eyebrows on the Alphatron Matte box keeping the sun out of the lens.

Some Matte Boxes like the Alphatron one shown here have small extra “eyebrows”. These are like mini flags that can be adjusted to provide extra shade for the lens. In the picture you can see how the shadow from the top eyebrow is keeping stray and unwanted light from falling on the lens. This will help minimise flare and preserve contrast in the images. It’s a small thing but it can make a big difference. Eyebrows and flags also keep light out of the matte box itself and help prevent reflections between any filters that you might use and the lens itself.

DSC05093-300x199 Matte Boxes and Lens Hoods can make your pictures sharper, also a look at the Alphatron Matte Box.
The Alphatron Matte Box can be fitted with a quick release swing away adapter to make lens changes quick and easy.

If you’re using prime lenses then you will probably need to change lenses regularly. A great time saver is the use of a swing-away adapter. The Alphatron Matte Box that I use has an optional quick release swing away mount option. By twisting a single lever the Matte Box opens up and swings away from the lens. This gives you easy access to the lens for cleaning or for a quick lens change without having to remove the Matte Box. When shooting out on location this is a big deal as there’s never anywhere clean to put your Matte Box when you want to do a lens swap.

DSC05075-e1397758050636-199x300 Matte Boxes and Lens Hoods can make your pictures sharper, also a look at the Alphatron Matte Box.
The Alphatron Matte Box has one fixed filter tray and one rotating tray.

Matte boxes can have both fixed and rotating filter trays or a combination of the two. Fixed trays are fine for ND filters and most diffusion filters. For graduated filters a rotating tray is preferable and for polarising filters a rotating tray is essential. The Alphatron Matte Box here has one fixed tray and one rotating tray. So I can use the fixed tray for any ND filters and then the rotating tray for grads or polarisers. I very nice feature of the Alphatron is a little recess in the very front of the sun shade and a little locking tab that allows you to put a safety glass in place in front of any filters to protect you filters and lens. This is very handy especially if your shooting something that could possibly splash on your expensive filters and damage the coatings.

So what filters should you get for your nice new Matte Box? First of all do be prepared to spend a little bit of money to get good quality filters. Filters can be plastic, resin or glass. Optical grade plastics and resins can make very good filters, but they tend to be prone to collecting dust through static electricity and they scratch easily. In addition if left in a hot car they can distort and warp. But, plastic and resin filters are light weight and normally a lot cheaper than the glass equivalent. Better quality filters will have anti-reflective coatings. A good quick test of the quality of any filter is to use a long focal length lens or zoomed-in zoom lens to check for distortions or focus issues introduced by the filter. You might not notice this at wide angles or zoomed out. So do check at longer focal lengths.

Good brands include Tiffen, Formatt and Schneider. These won’t be the cheapest on the market, but the quality is consistently good.  Filters come in different sizes, the most common is the 4×4 or 4″ by 4″. For longer focal lengths these are fine, but if you want to shoot at wider angles you may find that 4×4’s are not wide enough. The next size up is the 4″x 5″ but the next commonly used size is the 4″ x 5.65″ which is close to the old 4:3 TV aspect ratio. The extra width really helps when shooting wider shots in 16:9.

My most commonly used filters are ND filters. These help manage light when it’s too bright allowing you to use a smaller aperture to gain a shallower depth of field. If your using a CMOS camera you should use IR ND filters that cut not only the visible light but also infra red light that most CMOS cameras are sensitive to.

Next is a polarising filter. A circular polariser is great for reducing or controlling reflections from windows, cars etc, it’s also good for enhancing the contrast in clouds and the sky making the sky a richer, deeper blue. When using a polariser it needs to go in a rotating tray so you can turn it when composing your shot to alter the polarising effect.

Graduated ND filters are also useful to help deal with excessively bright sky. The top of the filter is a ND filter or coloured filter and the bottom is normally clear. By sliding the filter up and down within the matte box you can alter level where the brightness reduction takes place. A tobacco or orange coloured graduated filter can be used to create or enhance a sunset type look. Just watch for the graduation crossing through foreground objects in the scene which can give the game away and look odd.