Category Archives: PXW-FS5

cameras, PXW-FS5, Technology

FS5 Eclipse and 3D Northern Lights by Jean Mouette and Thierry Legault.

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Here is something a little different.

I few years ago I was privileged to have Jean Mouettee and Thierry Legault join me on one of my Northern Lights tours. They were along to shoot the Aurora on an FS100 (it might have been an FS700) in real time. Sadly we didn’t have the best of Auroras on that particular trip. Theirry is famous for his amazing images of the Sun with the International Space Station passing in front of it.

Amazing image by Thierry Legault of the ISS passing in front of the Sun.

Well the two of them have been very busy. Working with some special dual A7s camera rigs recording on to a pair of Atomos Shoguns, they have been up in Norway shooting the Northern Lights in 3D. You can read more about their exploits and find out how they did it here: https://www.swsc-journal.org/articles/swsc/abs/2017/01/swsc170015/swsc170015.html

To be able to “see” the Aurora in 3D they needed to place the camera rigs over 6km apart. I did try to take some 3D time-lapse of the Aurora a few years back with cameras 3Km apart, but that was timelapse and I was thwarted by low cloud. Jean and Thierry have gone one better and filmed the Aurora not only in 3D but also in real time. That’s no mean feat!

One of the two A7s camera rigs used for the real time 3D Aurora project. The next stage will use 4 cameras in each rig for whole sky coverage.

If you want to see the 3D movies take a look at this page: http://www.iap.fr/science/diffusion/aurora3d/aurora3d.html

I’d love to see these projected in a planetarium or other dome venue in 3D. It would be quite an experience.

Jean was also in the US for the total Eclipse in August. He shot the eclipse using an FS5 recording 12 bit raw on a Atomos Shogun. He’s put together a short film of his experience and it really captures the excitement of the event as well as some really spectacular images of the moon moving across the face of the sun. I really shows what a versatile camera the FS5 is.

If you want a chance to see the Northern Lights for yourself why not join me next year for one of my rather special trips to Norway. I still have some spaces. https://www.xdcam-user.com/northern-lights-expeditions-to-norway/

cinematography, PMW-F55, PXW-FS5, PXW-FS7, Technology

Why are Sony’s ISO’s different between standard gammas and log?

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With Sony’s log capable cameras (and most other manufacturers) when you switch between the standard gamma curves and log gamma there is a change in the cameras ISO rating. For example the FS7 is rated at 800 ISO in rec709 but rated at 2000 ISO in log. Why does this change occur and how does it effect the pictures you shoot?

As 709 etc has a limited DR (between around 6 and 10 stops depending on the knee settings) while the sensor itself has a 14 stop range, you only need to take a small part of the sensors full range to produce that smaller range 709 or hypergamma image. That gives the camera manufacturer some freedom to pick the sweetest part of the sensors range. his also gives some leeway as to where you place the base ISO.

I suspect Sony chose 800 ISO for the FS7 and F5 etc as that’s the sensors sweet spot, I certainly don’t think it was an accidental choice.

What is ISO on an electronic camera? ISO is the equivalent sensitivity rating. It isn’t a measure of the cameras actual sensitivity, it is the ISO rating you need to enter into a light meter if you were using an external light meter to get the correct exposure settings. It is the equivalent sensitivity. Remember we can’t change the sensor in these cameras so we can’t actually change the cameras real sensitivity, all we can do is use different amounts of gain or signal amplification to make the pictures brighter or darker.

When you go switch the camera to log you have no choice other than to take everything the sensor offers. It’s a 14 stop sensor and if you want to record 14 stops, then you have to take 100% of the sensors output. The camera manufacturer then chooses what they believe is the best exposure mid point point where they feel there is an acceptable compromise between noise, highlight and lowlight response. From that the manufacture will get an ISO equivalent exposure rating.

If you have an F5, FS7 or other Sony log camera, look at what happens when you switch from rec709 to S-Log2 but you keep your exposure constant.

Middle grey stays more or less where it is, the highlights come down. White will drop from 90% to around 73%. But the ISO rating given by the camera increases from 800ISO to 2000ISO. This increased ISO number implies that the sensor became more sensitive – This is not the case and a little missleading. If you set the camera up to display gain in dB and switch between rec709 (std gamma) and S-Log the camera stays at 0dB, this should be telling you that there is no change to the cameras gain, no change to it’s sensitivity. Yet the ISO rating changes – why?

The only reason the ISO number increases is to force us to underexpose the sensor by 1.3 stops (relative to standard gammas such as rec709 and almost every other gamma) so we can squeeze a bit more out of the highlights. If you were using an external light meter to set your exposure if you change the ISO setting on the light meter from 800 ISO to 2000 ISO  the light meter will tell you to close the aperture by 1.3 stops. So that’s what we do on the camera, we close the aperture down a bit to gain some extra highlight range.

But all this comes at the expense of the shadows and mid range. Because you are putting less light on the sensor if you use 2000 ISO as your base setting the shadows and mids are now not as good as they would be  in 709 or with the other standard gammas.

This is part of the reason why I recommend that you shoot with log between 1 and 2 stops brighter than the base levels given by Sony. If you shoot 1 stop brighter that is the equivalent to shooting at 1000 ISO and this is closer to the 800 ISO that Sony rate the camera at in standard gamma.  Shooting that bit brighter gives you a much better mid range that grades much better.

 

cinematography, PXW-FS5, Shooting Tips, shoots, Workflow

Why is exposing log brightly beneficial?

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I have been asked whether you should still expose log a bit brighter than the recommended base levels on the Sony PXW-FS5 now that Sony have released new firmware that gives it a slightly lower base ISO. In this article I take a look at why it might be a good idea to expose log (with any camera) a bit brighter than perhaps the manufacturer recommends.

There are a couple of reasons to expose log nice and bright, not just noise. Exposing log brighter makes no difference to the dynamic range. That’s determined by the sensor and the gain point at which the sensor is working. You want the camera to be at it’s native sensitivity or 0dB gain to get that maximum dynamic range.

Exposing brighter or darker doesn’t change the dynamic range but it does move the mid point of the exposure range up and down.  Exposing brighter increases the under exposure range but decreases the over exposure range. Exposing darker decreases the under exposure range but increases the over exposure range.

Something that’s important when thinking about dynamic range and big dynamic ranges in particular is that dynamic range isn’t just about the highlights it’s also about the shadows, it isn’t just over exposure, it’s under exposure too, it’s RANGE.

So why is a little bit of extra light often beneficial? You might call it “over exposure” but that’s not a term I like to use as it implies “too much exposure”. I prefer to use “brighter exposure”.

It’s actually quite simple, it’s about putting a bit more light on to the sensor. Most sensors perform better when you put a little extra light on them. One thing you can be absolutely sure of – if you don’t put enough light on the sensor you won’t get the best pictures.

Put more light on to the sensor and the shadows come up out of the sensors noise floor. So you will see further into the shadows. I’ve had people comment that “why would I ever want to use the shadows, they are always noisy and grainy”? But that’s the whole point – expose a bit brighter and the shadows will be much less noisy, they will come up out of the noise. Expose 1 stop brighter and you halve the shadow noise (for the same shadows at the previous exposure).  Shadows are are only ever noise ridden if you have under exposed them.

This is particularly relevant in controlled lighting. Say you light a scene for 9 stops. So you have 9 stops of dynamic range but a 14 stop sensor. Open up the aperture, put more light on the sensor, you get a better signal to noise ratio, less noisy shadows but no compromise of any type to the highlights because if the scene is 9 stops and you have 14 to play with, you can bring the exposure up by a couple of stops comfortably within the 14 stop capture range.

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. Note that current sensor only go to +6 stops ove middle grey so S-Log2 and S-Log record to different peak levels.

Look at the above diagram of Sony’s S-Log2 and S-Log3 curves. The vertical 0 line in the middle is middle grey. Note how above middle grey the log curves are more or less straight lines. That’s because above the nominal middle grey exposure level each stop is recorded with the same amount of data, this you get a straight line when you plot the curve against exposure stops. So that means that it makes very little difference where you expose the brighter parts of the image. Expose skin tones at stop + 1 or stop +3 and they will have a very similar amount of code values (I’m not considering the way dynamic range expands in the scene you shoot as you increase the light in the scene in this discussion). So it makes little difference whether you expose those skin tones at stop +1 or +3, after grading they will look the same.

Looking at the S-Log curve plots again note what happens below the “0” middle grey line. The curves roll off into the shadows. Each stop you go down has less data than the one before, roughly half as much. This mimics the way the light in a real scene behaves, but it also means there is less data for each stop. This is one of the key reasons why you never, ever want to be under exposed as if you are underexposed you mid range ends up in this roll off and will lack data making it not only noisy but also hard to grade as it will lack contrast and tonal information.

Open up by 1 additional stop and each of those darker stops is raised higher up the recording curve by one stop and every stop that was previously below middle grey doubles the amount of tonal values compared to before, so that’s 8 stops that will have 2x more data than before. This gives you a nice fat (lots of data) mid range that grades much better, not just because it has less noise but because you have a lot more data where you really need it – in the mid range.

Note: Skin tones can cover a wide exposure range, but typically the mid point is around 1 to 1.5 stops above middle grey. In a high contrast lighting situation skin tones will start just under middle grey and extend to about 2 stops over. If you accidentally under expose by 1 stop or perhaps don’t have enough light for the correct exposure you will seriously degrade the quality of your skin tones as half of your skin tones will be well below middle grey and in the data roll-off.

Now of course you do have to remember that if your scene does have a very large dynamic range opening up an extra stop might mean that some of the very brightest highlights might end up clipped. But I’d happily give up a couple of specular highlights for a richer more detailed mid range because when it comes to highlights – A: you can’t show them properly anyway because we don’t have 14 stop TV screens and B: because highlights are the least important part of our visual range.

A further consideration when we think about the highlights is that with log there is no highlight roll-off. Most conventional gamma curves incorporate a highlight roll-off to help increase the highlight range. These traditional highlight roll-offs reduce the contrast in the highlights as the levels are squeezed together and as a result the highlights contain very little tonal information. So even after grading they never look good, no matter what you do. But log has no highlight roll-off. So even the very brightest stop, the one right on the edge of clipping contains just as much tonal information as each of the other brighter than middle grey stops. As a result there is an amazingly large amount of detail than can be pulled out of these very bright stops, much more than you would ever be able to pull from most conventional gammas.

Compare log to standard gammas for a moment. Log has a shadow roll-off but no highlight roll-off. Most standard gammas have a strong highlight roll-off. Log is the opposite of standard gammas. With standard gammas, because of the highlight roll-off, we normally avoid over exposure because it doesn’t look good. With Log we need to avoid under exposure because of the shadow roll-off, it is the opposite to shooting with standard gammas.

As a result I strongly recommend you never, ever under expose log. I normally like to shoot log between 1 and 2 stops brighter than the manufacturers base recommendation.

Next week: Why is a Sony camera like the FS7,F5 800 ISO with standard gamma but 2000 ISO in log and how does that impact the image?

 

cinematography, PXW-FS5, PXW-FS7

The Pro’s and Con’s of 12 bit linear raw or recording raw to S-Log.

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UPDATE: JUST TO BE CLEAR, THERE IS NOTHING WRONG WITH SONY’S 12 BIT LINEAR RAW. BUT YOU REALLY SHOULD BE AWARE OF IT’S LIMITATIONS COMPARED TO 16 BIT RAW OR POSSIBLY EVEN 10 BIT LOG.

This came up in the comments today and it’s something that I get asked about quite a lot.

Sony’s high end cameras, designed for raw – F5, F55, F65 all use 16 bit linear data. This linear data contains an impressively large amount of picture information across the entire range from the darkest shadows to the brightest highlights. This huge amount of data gives footage that can be pushed and pulled in post all over the place. 16 bit raw gives you 65,536 discreet values.

The FS7 and FS5 use 12 bit linear raw. 12 bit data gives you 4096 discreet values, 1/15th of the values, a small fraction of what 16 bit has. This presents a problem as to record 14 stops with linear data you need more than 12 bits.

Not Enough Code Values.

There just aren’t enough code values with only 12 bits (which is why no one else does it). So Sony do some clever math to make it workable. This reduces the amount of tonal steps in in the shadows.  On it’s own this isn’t a huge problem, just make sure you expose brightly to avoid trying to pull to much info out of the shadows and definitely don’t use it for low light. On high key scenes 12 bit raw is very nice indeed, this is where it excels. On low key scenes it can appear very grainy, noisy and shadows often look coarse and lack smooth textures. Expose nice and bright and you will get great highly gradable footage. Expose dark and you will have big problems.

Transcoding can add to the problems.

Where you really can run into problems is if you take 12 bit raw (with it’s reduced shadow data) and convert that to 10 bit log (which has reduced highlight data relative to the scene you are shooting).

What you end up with is 10 bit log with reduced shadow data compared to a straight 10 bit log recording. If you compare the direct 10 bit S-log from an FS7 (or F5/F55) to 10 bit S-log derived from 12 bit raw from an FS5, the FS7 internal 10 bit log picture will have a little more shadow information while the highlights from both will be similar. So the direct internal 10 bit log recording from an FS7 will typically be a little better in the shadows than any log created from the 12  bit raw and there will little, if any, highlight benefit.  It’s a little different for the FS5 as in UHD this is limited to 8 bit, so the raw, converted to log from the FS5 will have better highlight data, but still be a touch weak in the darkest shadow areas.  Overall  10bit log derived from 12 bit linear will be better than 8 bit log, but not better than 10 bit log.

If 4K S-Log is really important to you – get an FS7, F5 or F55.

So I’d much rather have an FS7 (F5 or F55) if I want to shoot UHD or 4K S-log. That’s what these cameras are designed for. But, if you only have an FS5, the raw to log workflow will outperform the limited 8 bit UHD log, so it is still definitely beneficial for FS5 owners to shoot raw and convert it to 10 bit S-Log with an external recorder. But better still record raw, then you really will have a better image.

Raw with the FS7.

On the FS7 the benefits of recording 12 bit raw over 10 bit S-Log are less clear. For bright, well exposed scenes the 12 bit raw will  have a definite  edge. For low light or under exposed scenes there is no benefit to shooting 12 bit raw you will get nothing extra.

On the FS7 it is not a good idea to take the 12 bit raw output and record it as 10 bit S-Log on an external recorder. While you may have a less compressed codec, you will be compromising the shadows compared to the cameras own internally generated 10 bit log recordings.  In most cases you would be better off simply taking the HDMI output and recording that as it avoids the 12 bit linear shadow bottleneck.

Again though – exposing nice and bright is the key to a good result. Get the data up into the brighter parts of the recording and the raw can be fantastic.

Internal and external log brightness shifts.

When you record S-Log internally on the Sony cameras the recordings use full range data levels to maximise the codec performance. You can use data range (which exceeds the normal video range) as it is assumed the data will be graded and as part of this process restored to video range data for viewing. However when recording on an external recorder the recordings sometimes use full video range rather than data range or if it’s data range don’t have the right metadata. This shouldn’t be a huge problem if the grading software behaves itself and treats each type of content correctly, shifting each to one unified range, but sadly this is rarely the case (especially with Adobe). So not only do the internal and externally recorded images come out with different brightness and contrast, but also LUT’s designed for one don’t work the same with the other. It’s a bit of a minefield to be honest and one of the reason why I prefer to always grade with dedicated grading software like resolve which handles the levels conversions properly (most of the time at least).

cameras, PXW-FS5

PXW-FS5 Firmware Version 4.02 Released.

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Sony have just released firmware version 4.02 for the PXW-FS5. This firmware fixes the bugs found by Sony in the initial release of the version 4 firmware and includes the new Hybrid Log Gamma picture profile No. 10 along with a change to the cameras base ISO rating. I note that there is no mention of the problems with HLG clips in Adobe Premiere, so this will require further testing to see if this has been fixed.

The firmware can be downloaded from here:

https://www.sony.co.uk/pro/support/software/SW_122115_PSG/50

From Sony:

Ver4.02 (Functionally, it is the same as the Ver.4.00.)

V4.02 fixes the following issue:
1.      Video image may be recorded with short delay of 2 or 3 frames of audio in other recording modes than AVCHD.
2.      When choosing [HLG1],[HLG2] or [HLG3] in the PictureProfile and CENTER SCAN in the CAMERA/PAINT menu, rebooting the camera may cause brightness and color shift.

Ver4.00(For your information)
1.    Support for High Dynamic Range (HDR) by shooting in Hybrid Log-Gamma** (HLG) standard
2.    Support for continuous 120fps High Frame Rate (HFR) recording in 1080p with CBKZ-FS5HFR (sold separately)
3.    Option to change the minimum ISO sensitivity number to ISO 2000 from ISO 3200 when recording S-Log2/S-Log3

cameras, cinematography, HDR, PXW-FS5, Technology, Workflow

What is HLG and what is it supposed to be used for?

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While we wait for Sony to re-release the version 4 firmware for the FS5 I thought I would briefly take a look at what HLG is and what it’s designed to do as there seems to be a lot of confusion.

HLG stands for Hybrid Log Gamma. It is one of the gamma curves used for DISTRIBUTION of HDR content to HDR TV’s that support the HLG standard. It was never meant to be used for capture, it was specifically designed for delivery.

As the name suggests HLG is a hybrid gamma curve. It is a hybrid of Rec-709 and Log. But before you get all excited by the log part, the log used by HLG is only a small part of the curve and it is very agressive – it crams a very big dynamic range into a very small space – This means that if you take it into post production and start to fiddle around with it there is a very high probability of problems with banding and other similar artefacts becoming apparent.

The version of HLG in the FS5 firmware follows the BBC HLG standard (there is another NHK standard). From black to around 70% the curve is very similar to Rec 709, so from 0 to 70% you get quite reasonable contrast. Around 70% the curve transitions to a log type gamma allowing a dynamic range much greater than 709 to be squeezed into a conventional codec. The benefit this brings is that on a conventional Rec-709 TV the picture doesn’t look wrong. It looks like a very slightly darker than normal, only slightly flat mid range, but the highlights are quite flat and  washed out. For the average home TV viewer watching on a 709 TV the picture looks OK, maybe not the best image ever seen, but certainly acceptable.

However feed this same signal to an HDR TV that supports HLG and the magic starts to happen. IF the TV supports HLG (and currently only a fairly small proportion of HDR TV’s support HLG. Most use PQ/ST2084) then the HLG capable HDR TV will take the compressed log highlight range and stretch it out to give a greater dynamic range display. The fact that the signal gets stretched out means that the quality of the codec used is critical. HLG was designed for 10 bit distribution using HEVC, it was never meant to be used with 8 bit codecs, so be very, very careful if using it in UHD with the FS5 as this is only 8 bit.

So, HLG’s big party trick is that it produces an acceptable looking image on a Rec-709 TV, but also gives an HDR image on an HDR TV. So one signal can be used for both HDR and SDR giving what might be called backwards compatibility with regular SDR TV’s. But it is worth noting that on a 709 TV HLG images don’t look as good as images specifically shot or graded for 709. It is a bit of a compromise.

What about the dynamic range? High end HDR TV’s can currently show about 10 stops. Lower cost HDR TV’s may only be able to show 8 stops (compared to the 6 stops of a 709 TV). There is no point in feeding a 14 stop signal to a 10 stop TV, it won’t look the best. From what I’ve seen of the HLG curves in the FS5 they allow for a maximum of around 10 to 11 stops, about the same as the cinegammas. HLG can be used for much greater ranges, but as yet there are no TV’s that can take advantage of this and it will be a long tome before there are. So for now, the recorded range is a deliberately limited so you don’t see stuff in the viewfinder that will never be seen on todays HDR TV’s.  As a result the curves don’t use the full recording range of the camera. This means they are not using the recording data in a particularly efficient way, a lot of data is unused and wasted. But this is necessary to make the curves directly compatible with an HLG display.

What about grading them? My advice – don’t try to grade HLG footage. There are three problems. The first is that the gamma is very different in the low/mid range compared to the highlights. This means that in post the shadows and mid range will respond to corrections and adjustments very differently to the high range. That makes grading tricky as you need to apply separate correction to the midrange and highlights.

The second problem is that the is a very large highlight range squeezed into a very small recording range. It should look OK when viewed directly with no adjustment. But if you try stretching that out to make the highlights brighter (remember they never reach 100% as recorded) or to make them more contrasty, there is a higher probability of seeing banding artefacts than with any other gamma in the camera.

The third issue is simply that the limited recording range means you have fewer code values per stop than regular Rec-709, the cinegammas or S-Log2. HLG is the least best choice for grading in the FS5.

Next problem is color. Most HDR TV’s want Rec-2020 color. Most conventional monitors want Rec-709 color. Feed Rec-2020 into a 709 monitor and the colors look flat and the hues are all over the place, especially skin tones. Some highly saturated colors on the edge of the color gamut may pop out more than others and this looks odd.

Feed 709 into a 2020 TV and it will look super saturated and once again the color hues will be wrong. Also don’t fool yourself into thinking that by recording Rec2020 you are actually capturing more. The FS5 sensor is designed for 709. The color filters on the sensor do work a little beyond 709, but nowhere near what’s needed to actually “see” the full 2020 color space. So if you set the FS5 to 2020 what you are capturing is only marginally greater than 709. All you really have is the 709  with the hues shifted and saturation reduced so color looks right on a 2020 monitor or TV.

So really, unless you are actually feeding an Rec 2100 (HLG + 2020) TV, there is no point in using 2020 color as this require you to grade the footage to get the colors to look right on most normal TV’s and monitors. As already discussed, HLG is far from ideal for grading, so better to shot 709 if that’s what your audience will be using.

Don’t let the hype and fanfares that have surrounded this update cloud your vision. HLG is certainly very useful if you plan to directly feed HDR to a TV that supports HLG. But if you plan on creating HDR content that will be viewed on both HLG TV’s and the more common PQ/ST2084 TV’s then HLG is NOT what you want. You would be far – far better off shooting with S-Log and then grading your footage to these two very different HDR standards. If you try to convert HLG to PQ it is not going to look nearly as good as if you start with S-Log.

Exposure levels: If you want to get footage that works both with an HLG HDR TV and a SDR 709 TV then you need to expose carefully. A small bit of over exposure wont hurt the image when you view it on a 709 TV or monitor, so it will look OK in the viewfinder. But on an HDR TV any over exposure could result in skin tones that look much too bright and an image that is unpleasantly bright. As a guide you should expose diffuse 90% white (a white card or white piece of paper) at no more than 75%. Skin tones should be around 55 to 60%. You should not expose HLG as brightly as you do Rec-709.

Sure you can shoot with HLG for non HDR applications. You will get some slightly flat looking footage with rolled off highlights. If that’s the image you want then I’m not going to stop you shooting that way. If that’s what you want I suggest you consider the Cinegamma as these capture a similar DR also have a nice highlight roll off (when exposed correctly) and do use the full recording range.

Whatever you do make sure you understand what HLG was designed for. Make sure you understand the post production limitations and above all else understand that it absolutely is not a substitute for S-log.

cameras, PXW-FS5, Technology

PXW-FS5, Version 4.0 and above base ISO – BEWARE if you use ISO!!

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The new version 4.0 firmware for the PXW-FS5 brings a new lower base ISO range to the camera. This very slightly reduces noise levels in the pictures. If you use “gain” in dB to indicate your gain level, then you shouldn’t have any problems, +6dB is still +6dB and will be twice as noisy as 0dB. However if you use ISO to indicate your gain level then be aware that as the base sensitivity is now lower, if you use the same ISO with version 4 as you did with version 3 you will be adding more gain than before.

Version 3 ISO  in black, version 4 ISO in Blue

Standard 1000 ISO – 800 ISO
Still 800 ISO- 640 ISO
Cinegamma 1  800 ISO – 640 ISO
Cinegamma 2  640 ISO – 500 ISO
Cinegamma 3  1000 ISO – 800 ISO
Cinegamma 4  1000 ISO – 800 ISO
ITU709 1000 ISO – 800 ISO
ITU709(800) 3200 ISO – 2000 ISO
S-Log2 3200 ISO – 3200/2000 ISO
S-Log3 3200 ISO- 3200/2000 ISO

At 0dB or the base ISO these small changes (a little under 3dB) won’t make much difference because the noise levels are pretty low in either case. But at higher gain levels the difference is more noticeable.

For example if you  often used Cinegamma 1 at 3200 ISO with Version 3 you would be adding 12dB gain and the pictures would be approx 4x noisier than the base ISO.

With Version 4, 3200 ISO with Cinegamma 1 is an extra 15dB gain and you will have pictures approx 6 time noisier than the base ISO.

Having said that, because 0dB in version 4 is now a little less noisy than in version 3, 3200 ISO in V3 looks quite similar to 3200 ISO in version 4 even though you are adding a bit more gain.

PXW-FS5, Technology

What’s the difference between raw and S-Log ProRes – Re: FS5 raw output.

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This is a question that comes up a lot.

Raw is the unprocessed (or minimally processed) data direct from the sensor. It is just the brightness value for each of the pixels, it is not a color image, but we know which color filter is above each pixel, so we are able to work out the color later. In the computer you take that raw data and convert it into a conventional color video signal defining the gamma curve and colorspace in the computer.  This gives you the freedom to choose the gamma and colorspace after the shoot and retains as much of the original sensor information as possible.Of course the captured dynamic and color range is determined by the capabilities of the sensor and we can’t magically get more than the sensor can “see”. The quality of the final image is also dependant on the quality of the debayer process in the computer, but as you have the raw data you can always go back and re-encode the footage with a better quality encoder at a later date. Raw can be compressed or uncompressed. Sony’s 12 bit FS-raw when recorded on an Odyssey or Atomos recorder is normally uncompressed so there are no additional artefacts from compression, but the files are large. The 16 bit raw from a Sony F5 or F55 when recorded on an R5 or R7 is made about 3x smaller through a proprietary algorithm.

ProRes is a conventional compressed color video format. So a ProRes file will already have a pre-determined gamma curve and color space, this is set in the camera through a picture profile, scene file or other similar settings at the time of shooting. The quality of the ProRes file is dependant on the quality of the encoder in the camera or recorder at the time of recording, so there is no way to go back and improve on this or change the gamma/colorspace later. In addition ProRes, like most commonly used codecs is a lossy compressed format, so some (minimal) picture information may be lost in the encoding process and artefacts (again minimal) are added to the image. These cannot easily be removed later, however they should not normally present any serious problems.

It’s important to understand that there are many different types of raw and many different types of ProRes and not all are equal. The FS-raw from the FS5/FS7 is 12 bit linear and 12 bit’s are not really enough for the best possible quality from a 14 stop camera (there are not enough code values so floating point math and/or data rounding has to take place and this effects the shadows and low key areas of the image). You really need 16 bit data for 14 stops of dynamic range with linear raw, so if you are really serious about raw you may want to consider a Sony F5 or F55. ProRes is a pretty decent codec, especially if you use ProResHQ and 10 bit log approaches the quality of 12 bit linear raw but without the huge file sizes.  Incidentally there is very little to be gained by going to ProRes 444 when recording the 12 bit raw from an FS5/FS7, you’ll just have bigger files and less record time.

Taking the 12 bit raw from an FS5 and converting it to ProRes in an external recorder has potential problems of it’s own. The quality of the final file will be dependant on the quality of the debayer and encoding process in the recorder, so there may be differences in the end result from different recorders. In addition you have to add a gamma curve at this point so you must be careful to choose the correct gamma curve to minimise concatenation where you add the imperfections of 12 bit linear to the imperfections of the 10 bit encoded file (S-Log2 appears to be the best fit to Sony’s 12 bit linear raw).

Despite the limitations of 12 bit linear, it is normally a noticeable improvement over the FS5’s 8 bit internal UHD recordings, but less of a step up from the 10 bit XAVC that an FS7 can record internally. What it won’t do is allow you to capture anything extra. It won’t improve the dynamic range, won’t give you more color and won’t enhance the low light performance (if anything there will be a slight increase in shadow noise and it may be slightly inferior in under exposed shots). You will have the same dynamic and color range, but recorded with more “bits” (code values to be precise). Linear raw excels at capturing highlight information and what you will find is that compared to log there will be more textures in highlights and brighter parts of your captured scenes. This will become more and more important as HDR screens are better able to show highlights correctly. Current standard dynamic range displays don’t show highlights well, so often the extra highlight data in raw is of little benefit over log. But that’s going to change in the next few years so linear recording with it’s extra highlight information will become more and more important.

cameras, cinematography, PXW-FS5, Technology

ISO Confusion Once Again!

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I’m going to keep bringing this up until people start to take note and understand that with an electronic camera ISO is NOT sensitivity.

With an electronic camera ISO is a guide to the required shutter speed and aperture needed to get the correct exposure. This is different to sensitivity. The ISO rating of a video camera and it’s sensitivity are closely related, but they are not quite the same thing. Because different gamma curves require different exposures the ISO rating for each gamma curve will be different even though the gain and actual sensitivity of the camera may be exactly the same.

Lets take the  Sony PXW-FS5 as an example.

If you shoot using the standard camera settings you should expose white at 90%, middle grey will be around 42% and skin tones typically around 70%. At 0dB gain the camera the camera will display an ISO equivalent rating of 1000 ISO. So let’s say you are using a light meter. You set it to 1000 ISO and it tells you you need an aperture of f5.6 to get the right exposure.

Now you change to S-Log2. If you do nothing else your white card will now be at around 75% and middle grey will be around 40%. At 0dB gain the camera will show an equivalent ISO of 3200 ISO.

But hang on – The camera is still at 0dB gain, so there is no change in sensitivity. .But the camera is over exposed, S-Log2 is supposed to be exposed with white at 59% and middle grey at 32%.

So we go to our light meter and change the ISO on the light meter from 1000 ISO to 3200 ISO. Because the light meter now “thinks” the camera is more sensitive by almost 2 stops it will tell us to close the aperture by nearly 2 stops. So we go to the camera and stop down to f10 and bingo, the image is exposed correctly.

But here’s the important thing – The camera hasn’t become any more sensitive. We haven’t replaced the sensor with a different, more sensitive one (as you would do with a film camera where you actually change the film stock). We are still at 0dB gain (even though the camera tells us this is the equivalent to a higher ISO).

The only reason that ISO number changes is so that if we were using an external light meter we would get the recommended exposure levels for the gamma curve we are using. In this example closing the aperture increase the highlight range that the camera would be able to cope with and this helps us get that full 14 stop range from the camera, although closing the aperture means less light on the sensor so the pictures end up a little noisier as a result – That is unless you choose to rate the camera at a different ISO by over exposing the log a bit.

ISO is useful, but you need to understand that it isn’t really sensitivity. After all we can’t change the sensors on our video cameras and that would be the only way to truly change the sensitivity. Any “sensitivity” change is really nothing more than a gain or amplification change. Useful but not the same as changing the actual sensitivity. Gain will make a dark picture brighter but it won’t allow you to see something that the sensor can’t detect.

It is much easier to understand dB gain with an electronic camera as it actually tells you exactly what the camera is doing and it is actually my recommendation that people use gain rather than ISO for all of the above reasons.  The use of ISO on electronic cameras is very badly understood, in part because it’s a largely meaningless term because it doesn’t tell us how sensitive the sensor is, how much gain we are using or how much noise we are adding. Give any experienced camera operator a camera and ask them how noisy will it be a 18dB gain and they will have a pretty good idea of what the pictures will look like. Give them the same camera and ask them how noisy will it be at 8000 ISO and they won’t have a clue.

The problem is ISO is trendy and fashionable as that’s what “cinematographers” use. But lets be honest with ourselves – we are using electronic video cameras, whether that’s a Red, Alexa or FS5 so really we should be using the correct terminology for an electronic camera which is gain. It would eliminate an aweful lot of confusion and tell us how much noise and grain our pictures will have. It’s noise and grain will levels will determine how good a clip looks and how much we can grade it, so we need to clearly understand how much gain is being added in camera and dB gian tells us this. ISO does not.

Side Note: Modern film stocks will often have 2 ratings, the ISO or actual measured sensitivity of the film stock plus the EI or Exposure Index which is the recommended setting for the light meter to get the best exposure. In some respects the ISO rating of a video camera is closer to the EI rating of a film stock. Perhaps we should stop calling it ISO and use the term EI instead, this would be me appropriate and signify that it is a reference for best exposure rather than true sensitivity.

UPDATE: A comment on facebook was why not display both ISO and Gain side by side. This is an obvious solution really. Why do camera manufacturers force us to choose either ISO or gain? Why can’t we use a hybrid of the 2? I see no technical reason why cameras can’t show both the gain and ISO at the same time – Problem solved.