Even though I have written about these many times before the message still just doesn’t seem to be getting through to people.
Since the dawn of photography and video the only way to really change the signal to noise ratio and ultimately how noisy the pictures are is by changing how much light you put onto the sensor.
Gain, gamma, log, raw, etc etc only have a minimal effect on the signal to noise ratio. Modern cameras do admittedly employ a lot of noise reduction processes to help combat high noise levels, but these come at a price. Typically they soften the image or introduce artefacts such as banding, smear or edge tearing. So you always want to start off with the best possible image from the sensor with the least possible noise and the only way to achieve that is through good exposure – putting the optimum amount of light onto the sensor.
ISO is so confusing:
But just to confuse things the use of ISO to rate an electronic cameras sensitivity has become normal. But the problem is that most people have no clue about what this really means. On an electronic camera ISO is NOT a sensitivity measurement, it is nothing more than a number that you can put into an external light meter to allow you to use that light meter to obtain settings for the shutter speed and aperture that will give you the camera manufacturers suggest optimum exposure. That’s it – and that is very different to sensitivity.
Lets take Sony’s FS7 as an example (most other cameras behave in a very similar way).
If you set the FS7 up at 0dB gain, rec-709, it will have an exposure rating of 800 ISO. Use a light meter to expose with the meters ISO dial set to 800. Lets say the light meter says set the aperture to f8. When you do this the image is correctly exposed, looks good (well as good as 709 gets at least) and for most people has a perfectly acceptable amount of noise.
Now switch the camera to S-Log2 or S-Log3. With the camera still set to 0dB the ISO rating changes to 2000 which give the impression that the camera may have become more sensitive. But did we change the sensor? No. Have we added any more gain? No, we have not, the camera is still at 0dB. But if you now expose at the recommended levels, after you have done your grading and you grade to levels similar to 709 the pictures will look quite a lot noisier than pictures shot using Rec-709.
So what’s going on?
If you now go back to the light meter to expose the very same scene, you turn the ISO dial on the light meter from 800 to 2000 ISO and the light meter will tell you to now set the aperture to f13 (approx). So starting at the f8 you had for 800 ISO, you close the aperture on the camera by 1.3 stops to f13 and you will have the “correct” exposure.
BUT: now you are putting 1.3 stops less light on to the sensor so the signal coming from the sensor is reduced by 9dB and as a result the sensor noise that is always there and never really changes is much more noticeable. As a result compared to 709 the graded S-Log looks noisy and it looks noisier by the equivalent of 9dB. This is not because you have changed the cameras sensitivity or changed because you have changed the amount of camera gain but because compared to when you shoot in 709 the sensor is being under exposed and as a result it is outputting a signal 9dB lower. So in post production when you grade or add a LUT you have to add 9dB of gain to get the same brightness as the original direct rec-709 recording and as well as making the desirable image brighter it also makes the noise 9dB higher (unless you do some very fancy noise reduction work in post).
So what do you do?
It’s common simply to open the aperture back up again, typically by 1.5 stops so that after post production grading the S-log looks no more noisy than the 709 from the FS7 – Because in reality the FS7’s sensor works best for most people when rated at the equivalent of 800 ISO rather than 2000 – probably because it’s real sensitivity is 800 ISO.
When you think about it, when you shoot with Rec-709 or some other gamma that won’t be graded it’s important that it looks good right out of the camera. So the camera manufacturer will ensure that the rec-709 noise and grain v sensitivity settings are optimum – so this is probably the optimum ISO rating for the camera in terms of noise, grain and sensitivity.
So don’t be fooled into thinking that the FS7 is more sensitive when shooting with log, because it isn’t. The only reason the ISO rating goes up as it does is so that if you were using a light meter it would make you put less light onto the sensor which then allows the sensor to handle a brighter highlight range. But of course if you put less light onto the sensor the sensor won’t be able to see so far into the shadows and the picture may be noisy which limits still further the use of any shadow information. So it’s a trade-off, more highlights but less shadows and more noise. But the sensitivity is actually the same. Its’s an exposure change not a sensitivity change.
So then we get into the S-Log2 or S-Log3 debate.
First of all lets just be absolutely clear that both have exactly the same highlight and shadow ranges. Both go to +6 stops and -8 stops, there is no difference in that regard. Period.
And lets also be very clear that both have exactly the same signal to noise ratios. S-log3 is NOT noisier than S-log2. S-log 3 records some of the mid range using higher code values than S-Log2 and before you grade it that can sometimes make it appear like it’s noisier, but the reality is, it is not noisier. Just like the differing ISO ratings for different gamma curves, this isn’t a sensitivity change, it’s just different code values being used. See this article if you want the hard proof: http://www.xdcam-user.com/2014/03/understanding-sonys-slog3-it-isnt-really-noisy/
Don’t forget when you shoot with log you will be grading the image. So you will be adjusting the brightness of the image. If you grade S-Log2 and S-Log3 to the same brightness levels the cumulative gain (the gain added in camera and the gain added in post) ends up the same. So it doesn’t matter which you use in low light the final image, assuming a like for like grade will have the same amount of noise.
For 8 bit records S-Log2 has different benefits.
S-Log2 was designed from the outset for recording 14 stops with an electronic video camera. So it makes use of the cameras full recording range. S-Log3 is based on an old film log curve (cineon) designed to transfer 16 stops or more to a digital intermediate. So when the camera only has a 14 stop sensor you waste a large part of the available recording range. On a 10 bit camera this doesn’t make much difference. But on a 8 bit camera where you are already very limited with the number of tonal values you can record it isn’t ideal and as a result S-Log2 is often a better choice.
But if I shoot raw it’s all going to be so much better – isn’t it?
Yes, no, maybe…. For a start there are lot’s of different types of raw. There is linear raw, log raw, 10 bit log raw, 12 bit linear, 16 bit linear and they are all quite different.
But they are all limited by what the sensor can see and how noisy the sensor is. So raw won’t give you less noise (it might give different looking noise). Raw won’t give you a bigger dynamic range so it won’t allow you to capture deeper or brighter highlights.
But what raw does normally is to give you more data and normally less compression than the cameras internal recordings. In the case of Sony’s FS5 the internal UHD recordings are 8 bit and highly compressed while the raw output is 12 bit, that’s a 4 fold increase in the amount of tonal values. You can record the 12bit raw using uncompressed cDNG or Apples new ProResRaw codec which doesn’t introduce any appreciable compression artefacts and as a result the footage is much more flexible in post production. Go up to the Sony Venice, F5 or F55 cameras and you have 16 bit raw and X-OCN (which behaves exactly like raw) which has an absolutely incredible range of tonal values and is a real pleasure to work with in post production. But even with the Venice camera the raw does not have more dynamic range than the log. However because there are far more tonal values in the raw and X-OCN you can do more with it and it will hold up much better to aggressive grading.
It’s all about how you expose.
At the end of the day with all of these camera and formats how you expose is the limiting factor. A badly exposed Sony Venice probably won’t end up looking anywhere near as good as a well exposed FS7. A badly exposed FS7 won’t look as good as a well exposed FS5. No camera looks good when it isn’t exposed well.
Exposure isn’t brightness. You can add gain to make a picture brighter, you can also change the gamma curve to change how bright it is. But these are not exposure changes. Exposure is all about putting the optimum amount of light onto the sensor. Enough light to produce a signal from the sensor that will overcome the sensors noise. But also not so much light that the sensor overloads. That’s what good exposure is. Fiddling around with gamma curves and gain settings will only every make a relatively small difference to noise levels compared to good exposure. There’s just no substitute for faster lenses, reflectors or actually adding light if you want clean images.
And don’t be fooled by ISO ratings. They don’t tell you how noisy the picture is going to be, they don’t tell you what the sensitivity is or even if it’s actually changing. All it tells you is what to set a light meter to.
There are a lot of videos circulating on the web right now showing what appears to be some kind of magic trick where someone has shot over exposed, recorded the over exposed images using ProRes Raw and then as if by magic made some adjustments to the footage and it goes from being almost nothing but a white out of over exposure to a perfectly exposed image.
This isn’t magic, this isn’t raw suddenly giving you more over exposure range than you have with log, this is nothing more than a quirk of the way FCP-X handles ProRes Raw material.
Before going any further – this isn’t a put-down of raw or ProRes raw. It’s really great to be able to take raw sensor data and record that with only minimal processing. There are a lot of benefits to shooting with raw (see my earlier post showing all the extra data that 12 bit raw can give). But a magic ability to let you over expose by seemingly crazy amounts isn’t something raw does any better than log.
Currently to work with ProRes Raw you have to go through FCP-X. FCP-X applies a default sequence of transforms to the Raw footage to get it from raw data to a viewable image. These all expect the footage to be exposed exactly as per the camera manufacturers recommendations, with no leeway. Inside FCP-X it’s either exposed exactly right, or it isn’t.
The default decode settings include a heavy highlight roll-off. Apple call it “Tone Mapping”. Fancy words used to make it sound special but it’s really no different to a LUT or the transforms and processes that take place in other raw decoders. Like a LUT it maps very specific values in the raw data to very specific output brightness values. So if you shoot just a bit bright – as you would often do with log to improve the signal to noise ratio – The ProRes raw appears to be heavily over exposed. This is because anything bright ends up crushed into nothing but flat white by the default highlight roll off that is applied by default.
In reality the material is probably only marginally over exposed, maybe just one to 2 stops which is something we have become used to doing with log. When you view brightly exposed log, the log itself doesn’t look over exposed, but if you apply a narrow high contrast 709 LUT to it, it then the footage looks over exposed until you grade it or add an exposure compensated LUT. This is what is happening by default inside FCP-X, a transform is being applied that makes brightly exposed footage look very bright and possibly over exposed – because thats the way it was shot!
This is why in FCP-X it is typical to change the color library to WCG (Wide Color Gamut) as this changes the way FCP-X processes the raw, changing the Tone Mapping and most importantly getting rid of the highlight roll off. With no roll-off, highlights and any even slight over exposure will still blow out as you can’t show 14 stops on a conventional 6 stop TV or monitor. Anything beyond the first 6 stops will be lost, the image will look over exposed until you grade or adjust the material to control the brighter parts of the image and bring them back into a viewable range. When you are in WCG mode in FCP-X the there is no longer a highlight roll off crushing the highlights and now because they are not crushed they can be recovered, but there isn’t any more highlight range than you would have if you shot with log on the same camera!
None of this is some kind of Raw over exposure magic trick as is often portrayed. It’s simply not really understanding how the workflow works and appreciating that if you shoot bright – well it’s going to look bright – until you normalise it in post. We do this all the time with log via LUT’s and grading too! It can be a little more straight forward to recover highlights from Linear Raw footage as comes form an FS5 or FS7 compared to log. That’s because of the way log maintains a constant data level in each highlight stop and often normal grading and colour correction tools don’t deal with this correctly. The highlight range is there, but it can be tricky to normalise the log without log grading tools such as the log controls in DaVinci Resolve.
Another problem is the common use of LUT’s on log footage. The vast majority of LUT’s add a highlight roll off, if you try to grade the highlights after adding a LUT with a highlight roll off it’s going to be next to impossible to recover the highlights. You must do the highlight recovery before the LUT is added or use a LUT that has compensation for any over exposure. All of these things can give the impression that log has less highlight range than the raw from the same camera. This is not normally the case, both will be the same as it’s the sensor that limits the range.
The difference in the highlight behaviour is in the workflows and very often both log and raw workflows are miss-understood. This can lead to owners and users of these cameras thinking that one process has more than the other, when in reality there is no difference, it’s appears to be different because the workflow works in a different way.
This just keeps coming up over and over. Almost all log gamma curves and the majority of raw recording formats don’t have a highlight roll-off. Any roll off that you might see is probably in the LUT’s that you are using.
The whole point of log and raw is to capture as much information about the scene that you are shooing as you can. Log normally achieves this by recording every stop above middle grey with a constant amount of data, so even the very brightest stop has the same amount of recording data as the ones below it – there is no roll off.
In conventional limited range recordings such as Rec-709, hypergamma, cinegamma etc, highlight roll-offs work by reducing the contrast in the highlights to make the amount of data needed to record the very brightest stops much smaller than used for the rest of the image. This allows 2 or 3 stops to be squeezed into a very small recording range, keeping most of the recording data available for a nice bright high contrast image. The reduction in contrast in the extreme highlights helps hide any highlight handling problems and makes it appear as though the sensors clipping point is reach in a more pleasing soft manner.
But you don’t want this in a log or raw recording as it makes grading much harder as the footage will contain different contrast ranges, each needing it’s own grading adjustments. Also by reducing contrast in the highlights you are reducing the data. It would be very difficult to un-pick a highlight roll off and if you did want to expand the data back out you will get issues such as banding.
S-Log2 and S-Log3 like almost all log gammas have no highlight roll-off. The only roll off is from middle grey and down. So if you underexpose you will start to roll away the data in your scenes mid range and that’s not good. Expose for the mid range, this is the most important part of any image. If your highlights are a bit clipped don’t worry about this too much. In post production you can add a roll off in the grade that will make any clipped highlights roll away gently. Adding a bit of highlight diffusion in post will also nicely mask any clipped highlights and make them look natural.
The PXW-FS5 is a pretty good camera overall. Compared to cameras from 6 or 7 years ago it’s actually pretty sensitive. The exposure rating of 800 ISO for the standard rec-709 picture profile tells us that it is a little more than twice as sensitive as most old school shoulder cams. But it also suggests that it is only around half as sensitive as the king of low light, the Sony A7S. The A7S is so sensitive because it’s sensor is 1.5x bigger than the sensor in the FS5 and as a result the pixels in the A7S are almost twice the size, so are able to capture more light.
So what can you do if shooting in low light?
The most important thing to do is to make the optical system as efficient as possible. You want to capture as much of the available light as you can and squeeze it down onto the FS5’s sensor. If you take a fast full frame lens and use it in conjunction with a Speed Booster type adapter you will end up with similar performance to using the same lens, without the speed booster on an A7S.
This is because the lens has a fixed light gathering capability. Use it on an A7S and all of the captured light is passed to all of those big pixels on the sensor. The light is split evenly across 4K’s worth of pixels.
Use it on an FS5 with a speedbooster and the same thing happens, all of the light is compressed down, which makes it brighter and all of this now brighter light falls on 4K’s worth of pixels. The smaller pixels are about half as sensitive, but now the light is twice as bright, so the end result is similar.
The biggest performance gains are to be had from using a very fast lens and then making sure all the light from the lens is used, none wasted. Anything slower than f2.8 will be a waste. If you are thinking of using the Sony f4 lens for very low light… well frankly you may as well not bother. The lens is THE most important factor in low light. When I go up to Norway to shoot the Aurora I use f1.4 and f1.8 lenses.
What about Picture Profiles?
The standard picture profile isn’t a bad choice for low light but you might want to look at using cinegamma 3. Although with a low light, low contrast scene none of the picture profiles will be significantly different from the others with the exception of PP2, PP7, PP8 or PP9. None of the profiles make the camera more sensitive, the sensitivity is governed by the sensor itself and all the profiles do is alter the way gain is distributed across the image.
PP2 will crush your shadows giving less to work with in post. The log curves in PP7,8,9 will roll off the darkest parts of the image, again giving you less in post. So I would probably avoid these.
For color I suggest using the Pro colour matrix. This works well for most situations and it will help limit the noise levels as it keeps the saturation fairly low keeping the noisy blue channel in check.
Gain or ISO?
I recommend you set the camera to gain rather than ISO as the ISO’s for each each gamma curve are different, so it can be difficult to understand how much gain is being added, especially if you are switching between gamma curves. Use gain and you will have a good idea of the noise levels as every time you add +6dB the image becomes one stop brighter and you double the noise in the image, +12 dB is 2 stops brighter and 4x noisier than 0dB etc. ISO is an exposure rating, it is not a sensitivity measurement. But don’t use too much gain or too high an ISO as this will affect you ability to use some of the very good post production noise reduction tools that are available.
Noise and Noise Reduction.
If shooting in very low light then you are quite probably going to want to use some noise reduction tools in post production. “Neat Video” works very well at cleaning up a noisy image as do the various NR tools in the paid versions of DaVinci Resolve. These post production tools work best when the noise is clean. By that I mean well defined. When using any of the 709 or Cinegamma curves a bit of gain can be used, but I wouldn’t go above 12dB as above this the NR starts to introduce a lot of smear and this than makes it hard for any post production NR processes like Neat Video to do a decent job without the image turning into a blurry mess. So don’t go crazy with the gain or use very high ISO’s as the post production NR won’t work as well on footage that already has a lot of in camera NR applied.
And if you can add a little light-
If you are adding any light use a daylight balanced light where you can. Video cameras are least sensitive in the blue channel. If you use a tungsten light which is predominantly warm/red to get a good white balance you have to increase the gain of the cameras least sensitive and as a result most noisy blue channel. This will add more noise than if you use a daylight balance light as for daylight you need less gain in the noisy blue channel.
There is no miracle cure for shooting in very low light levels. But with the right lens and a speedbooster the FS5 can do a very good job. But just in case yo haven’t worked it out already, I’ll say it one more time: The lens is the most important bit! Beyond this your next step would be adding an image intensifier for that green night vision look.
Last week I presented a Webinar in conjunction with Visual Impact on how to shoot ProRes Raw with Sony’s FS5 and FS7. The Webinar was recorded, so if you missed it you can now watch it online. It’s almost 2 hours long and contains what I hope is a lot of useful information including what you need, exposure and how to get the footage in to FCP-X. I tried to structure the FCP-X part presentation in such a way that those that don’t normally use FCP-X (like me) will be able to get started quickly and understand what is going on under the hood.
Since the webinar it has been brought to my attention by Felipe Baez (thanks Felipe) that it is possible to add a LUT after the color panel and grading tools by adding the “custom LUT” effect to your clip. To do this you will set the raw input conversion to S-log3. Then add your color correction, then add the Custom LUT effect.
A big thank you to Visual Impact for making this possible, do check them out!
Here is the link to the video of the webinar:
It’s interesting to see how the term viewfinder is now used for small monitors rather than monocular viewfinders or shrouded dedicated viewfinders. Unless the a monitor screen is properly shielded from external light then you can only guess at the contrast and brightness of the images feeding it in anything other than a dim/dark room.
This is one of the key reasons why for decades viewfinders have been in fully shrouded hoods, snoots or loupes. As one of the key roles of a viewfinder is to show how your recordings will look for exposure assessment, if it doesn’t have a full shroud then in my opinion it isn’t a viewfinder, it is simply a monitor and exactly what your images will look like is anyones guess depending on the ambient light conditions. Furthermore even a young person with perfect can’t focus properly at less than 6″/150mm and that distance increases with age or in low ambient light. So most people will need a loupe or magnifying lens to be able to make full use of a small HD LCD for critical focus. In order to be able to see the sharpness of an image you need contrast, so an unshaded LCD screen on a sunny day will be next to useless for focus – perhaps this is why I see so many out of focus exterior shots on TV these days?
To be truly useful a viewfinder needs to be viewed in a controlled and dark environment. That’s why for decades it has been normal to use a monocular viewfinder. The eyepiece creates a tightly controlled, nice and dark, viewing environment. This isn’t always convenient. I will often flip up or remove the eyepiece for certain types of shot. But – if you don’t have the option to fully shade the viewfinder – how do you work with it on a sunny day? On a camera like the FS5 I often find myself using the small, enclosed viewfinder on the back of the camera when the sun is bright. These tiny built in viewfinders are not ideal, but I’d rather have that than a totally washed out LCD or trying to shoot with a jacket over my head as my only option.
So next time you are looking at upgrading the monitor or viewfinder on your camera do try out a good 3rd party monocular viewfinder such as the Zacuto Gratical or Zacuto Eye. Perhaps consider a Small HD monitor with the Side Finder option. Or an add-on monocular for the existing LCD panel. Without that all important shading and magnification it isn’t really a viewfinder, it’s just a small LCD monitor and in anything other than a very dim environment it’s always going to be tough to judge focus and exposure.
The use o f the LC709 Type A LUT in Sony’s Cinealta cameras such as the PXW-FS7 or PMW-F55 is very common. This LUT is popular because it was designed to mimic the Arri cameras when in their Rec-709 mode. But before rushing out to use this LUT and any of the other LC709 series of LUT’s there are some things to consider.
The Arri cameras are rarely used in Rec-709 mode for anything other than quick turn around TV. You certainly wouldn’t normally record this for any feature or drama productions. It isn’t the “Arri Look” The Arri look normally comes as a result of shooting using Arri’s LogC and then grading that to get the look you want. The reason it exists is to provide a viewable image on set. It has more contrast than LogC and uses Rec 709 color primaries so the colors look right, but it isn’t Rec-709. It squeezes almost all of the cameras capture range into a something that can be viewed on a 709 monitor so it looks quite flat.
Because a very large dynamic range is being squeezed into a range suitable to be viewed on a regular, standard dynamic range monitor the white level is much reduced compared to regular Rec-709. In fact, white (such as a white piece of paper) should be exposed at around 70%. Skin tones should be exposed at around 55-60%.
If you are shooting S-Log on a Sony camera and using this LUT to monitor, if you were to expose using conventional levels, white at 85-90% skin tones at 65-70%, then you will be offsetting your exposure by around +1.5 stops. On it’s own this isn’t typically going to be a problem. In fact I often come across people that tell me that they always shoot at the cameras native EI using this LUT and get great, low noise pictures. When I dig a little deeper I often find that they are exposing white at 85% via the LC709 LUT. So in reality they are actually shooting with an exposure the equivalent of +1 to +1.5 stops over the base level.
Where you can really run into problems is when you have already added an exposure offset. Perhaps you are shooting on an FS7 where the native ISO is 2000 ISO and using an EI of 800. This is a little over a +1 stop exposure offset. Then if you use one of the LC709 LUT’s and expose the LUT so white is at 90% and skin tones at 70% you are adding another +1.5 stops to the exposure, so your total exposure offset is approaching 3 stops. This large an offset is rarely necessary and can be tricky to deal with in post. It’s also going to impact your highlight range.
So just be aware that different LUT’s require different white and grey levels and make sure you are exposing the LUT at it’s correct level so that you are not adding an additional offset to your desired exposure.