Atomos have release a firmware update for the Shogun 7 to correctly implement raw recording from the FX6 at up to 60fps.
AtomOS 10.43 is a free update for the Shogun 7 that enables up to DCI 4Kp60 full-frame ProRes RAW recording from Sony’s FX6 camera!
Now available: Apple ProRes RAW recording support for Sony’s new FX6 professional camera with full-frame image sensor; which natively outputs RAW over SDI. This is all made possible with the Atomos Shogun 7 AtomOS 10.43 update which now allows users to record pristine ProRes RAW images at up to DCI 4Kp60. The resulting images have amazing detail and a high degree of latitude to utilize in post-production – optimal for HDR finishing or to give greater flexibility in SDR (Rec.709).
Raw can be a brilliant tool, I use it a lot. High quality raw is my preferred way of shooting. But it isn’t magic, it’s just a different type of recording codec.
All too often – and I’m as guilty as anyone – people talk about raw as “raw sensor data” a term that implies that raw really is something very different to a normal recording. In reality it’s really not that different. When shooting raw all that happens is that the video frames from the sensor are recorded before they are converted to a colour image. A raw frame is still a picture, it’s just that it’s a bitmap image made up of brightness values, each pixel represented by a single brightness code value rather than a colour image where each location in the image is represented by 3 values one for each of Red, Green and Blue or Luma, Cb and Cr.
As that raw frame is still nothing more than a normal bitmap all the cameras settings such as white balance, ISO etc are in fact baked in to the recording. Each pixel only has one single value and that value will have been determined by the way the camera is setup. Nothing you do in post production can change what was actually recorded. Most CMOS sensors are daylight balanced, so unless the camera adjusts the white balance prior to recording – which is what Sony normally do – your raw recording will be daylight balanced.
Modern cameras when shooting log or raw also record metadata that describes how the camera was set when the image was captured.
So the recorded raw file already has a particular white balance and ISO. I know lots of people will be disappointed to hear this or simply refuse to believe this but that’s the truth about a raw bitmap image with a single code value for each pixel and that value is determined by the camera settings.
This can be adjusted later in post production, but the adjustment range is not unlimited and it is not the same as making an adjustment in the camera. Plus there can be consequences to the image quality if you make large adjustments.
Log can be also adjusted extensively in post too. For decades feature films shot on film were scanned using 10 bit Cineon log (which is the log gamma curve S-Log3 is based on) and 10 bit log used for post production until 12 bit and then 16 bit linear intermediates came along like OpenEXR. So this should tell you that actually log can be graded very well and very extensively.
But then many people will tell you that you can’t grade log as well as raw. Often they will give photographers as an example where there is a huge difference between what you can do with a raw photo and a normal image. But we also have to remember this is typically comparing what you can do with a highly compressed 8 bit jpeg file and an often uncompressed 12 or 14 bit raw file. It’s not a fair comparison, of course you would expect the 14 bit file to be better.
The other argument often given is that it’s very hard to change the white balance of log in post, it doesn’t look right or it falls apart. Often these issues are nothing to do with the log recording but more to do with the tools being used.
When you work with raw in your editing or grading software you will almost always be using a dedicated raw tool or raw plugin designed for the flavour of raw you are using. As a result everything you do to the file is optimised for the exact flavour of raw you are dealing with. It shouldn’t come as a surprise to find that to get the best from log you should be using tools specifically designed for the type of log you are using. In the example below you can see how Sony’s Catalyst Browse can perfectly correctly change the white balance and exposure of S-log material with simple sliders just as effectively as most raw formats.
Applying the normal linear or power law (709 is power law) corrections found in most edit software to Log won’t have the desired effect and basic edit software rarely has proper log controls. You need to use a proper grading package like Resolve and it’s dedicated log controls. Better still some form of colour managed workflow like ACES where your specific type of log is precisely converted on the fly to a special digital intermediate and the corrections are made to the intermediate file. There is no transcoding, you just tell ACES what the footage was was shot on and magic happens under the hood. Once you have done that you can change the white balance or ISO of log material in exactly the same way as raw. There is very, very little difference.
When people say you can’t push log, more often than not it isn’t a matter of can’t, it’s a case of can – but you need to use the right tools.
Less compression or a greater bit depth are where the biggest differences between a log or raw recording come from, not so much from whether the data is log or raw. Don’t forget raw is often recorded using log, which kind of makes the “you can’t grade log” argument a bit daft.
Camera manufactures and raw recorder manufacturers are perfectly happy to allow everyone to believe raw is magic and worse still, let people believe that ANY type of raw must be better than all other types of recordings. Read though any camera forum and you will see plenty of examples of “it’s raw so it must be better” or “I need raw because log isn’t as good” without any comprehension of what raw is and how in reality it’s the way the raw is compressed and the bit depth that really matters.
If we take ProRes Raw as an example: For a 4K 24/25fps file the bit rate is around 900Mb/s. For a conventional ProRes HQ file the bit rate is around 800Mb/s. So the file size difference between the two is not at all big.
But the ProRes Raw file only has to store around 1/3 as many data points as the component ProResHQ file. As a result, even though the ProRes Raw file often has a higher bit depth, which in itself usually means better a better quality recording, it is also much, much less compressed and as a result will have fewer artefacts.
It’s the reduced compression and deeper bit depth possible with raw that can lead to higher quality recordings and as a result may bring some grading advantages compared to a normal ProRes or other compressed file. The best bit is there is no significant file size penalty. So you have the same amount of data, but you data should be of higher quality. So given that you won’t need more storage, which should you use? The higher bit depth less compressed file or the more compressed file?
But, not all raw files are the same. Some cameras feature highly compressed 10 bit raw, which frankly won’t be any better than most other 10 bit recordings as you are having to do all the complex math to create a colour image starting with just 10 bit. Most cameras do this internally at at least 12 bit. I believe raw needs to be at least 12 bit to be worth having.
If you could record uncompressed 12 bit RGB or 12 bit component log from these cameras that would likely be just as good and just as flexible as any raw recordings. But the files would be huge. It’s not that raw is magic, it’s just that raw is generally much less compressed and depending on the camera may also have a greater bit depth. That’s where the benefits come from.
It’s amazing how often people will tell you how easy it is to change the white balance or adjust the ISO of raw footage in post. But can you, is it really true and is it somehow different to changing the ISO or white balance of Log footage?
Let’s start with ISO. If ISO is sensitivity, or the equivalent of sensitivity how on earth can you change the sensitivity of the camera once you get into post production. The answer is you can’t.
But then we have to consider how ISO works on an electronic camera. You can’t change the sensor in a video camera so in reality you can’t change how sensitive an electronic camera is (I’m ignoring cameras with dual ISO for a moment). All you can do is adjust the gain or amplification applied to the signal from the sensor. You can add gain in post production too. So, when you adjust the exposure or using the ISO slider for your raw footage in post all you are doing is adjusting how much gain you are adding. But you can do the same with log or any other gamma.
One thing that makes a difference with raw is that the gain is applied in such a way that what you see looks like an actual sensitivity change no matter what gamma you are transforming the raw to. This makes it a little easier to make changes to the final brightness in a pleasing way. But you can do exactly the same thing with log footage. Anything you do in post must be altering the recorded file, it can never actually change what you captured.
Changing the white balance in post: White Balance is no different to ISO, you can’t change in post what the camera captured. All you can do is modify it through the addition or subtraction of gain.
Think about it. A sensor must have a certain response to light and the colours it sees depending on the material it’s made from and the colour filters used. There has to be a natural fixed white balance or a colour temperature that it works best at.
The Silicon that video sensors are made from is almost always more sensitive at the red end of the spectrum than the blue end. So as a result almost all sensors tend to produce the best results with light that has a lot of blue (to make up for the lack of blue sensitivity) and not too much red. So most cameras naturally perform best with daylight and as a result most sensors are considered daylight balanced.
If a camera produces a great image under daylight how can you possibly get a great image under tungsten light without adjusting something? Somehow you need to adjust the gain of the red and blue channels.
Do it in camera and what you record is optimised for your choice of colour temperature at the time of shooting. But you can always undo or change this in post by subtracting or adding to whatever was added in the camera.
If the camera does not move away from its native response then if you want anything other than the native response you will have to do it in post and you will be recording at the cameras native white balance. If you want a different colour temp then you need to add or subtract gain to the R & B channels in post to alter it.
Either way what you record has a nominal white balance and anything you do in post is skewing what you have recorded using gain. There is no such thing as a camera with no native white balance, all cameras will favour one particular colour temperature. So even if a manufacturer claims that the white balance isn’t baked in what they mean is they don’t offer the ability to make any adjustments to the recorded signal. If you want the very best image quality, the best method is to adjust at the time of recording. So, as a result a lot of camera manufacturers will skew the gain of the red and blue channels of the sensor in the camera when shooting raw as this optimises what you are recording. You can then skew it again in post should you want a different balance.
With either method if you want to change the white balance from what was captured you are altering the gain of the red and blue channels. Raw doesn’t magically not have a white balance, so shooting with the wrong white balance and correcting it in post is not something you want to do. Often you can’t correct badly balanced raw any better than you can correct incorrectly balanced log.
How far you can adjust or correct raw depends on how it’s been compressed (or not), the bit depth, whether it’s log or linear and how noisy it is. Just like a log recording really, it all depends on the quality of the recording.
The big benefit raw can have is that the amount of data that needs to be recorded is considerably reduced compared conventional component or RGB video recordings. As a result it’s often possible to record using a greater bit depth or with much less compression. It is the greater bit depth or reduced compression that really makes a difference. 16 bit data can have up to 65,536 luma gradations, compare that to the 4096 of 12 bit or 1024 of 10 bit and you can see how a 16 bit recording can have so much more information than a 10 bit one. And that makes a difference. But 10 bit log v 10 bit raw, well it depends on the compression, but well compressed 10 bit log will likely outperform 10 bit raw as the all important colour processing will have been done in the camera at a much higher bit depth than 10 bit.
Hooray! Finally ProRes Raw is supported in both the Mac and Windows versions of Adobe Creative Cloud. I’ve been waiting a long time for this. While the FCP workflow is solid and works, I’m still not the biggest fan of FCP-X. I’ve been a Premiere user for decades although recently I have switched almost 100% to DaVinci Resolve. What I would really like to see is ProRes Raw in Resolve, but I’m guessing that while Black Magic continue to push Black Magic Raw that will perhaps not come. You will need to update your apps to the latest versions to gain the ProRes Raw functionality.
This comes up again and again, hence why I am writing about it once again.
Raw should never be converted to log before recording if you want any benefit from the raw. You may as well just record the 10 bit log that most cameras are capable of internally. Or take log and output it via the cameras 10 bit output (if it has one) and record that directly on the ProRes recorder. It doesn’t matter how you do it but if you convert between different recording types you will always reduce the image quality and this is as bad a way to do it as you can get. This mainly relates to cameras like the PXW-FS7. The FS5 is different because it’s internal UHD recordings are only 8 bit, so even though the raw is still compromised by converting it to ProRes log, this can still be better than the internal 8 bit log.
S-Log like any other log is a compromise recording format. Log was developed to squash a big dynamic range into the same sized recording bucket as would normally be used for conventional low dynamic range gammas. It does this by discarding a lot of tonal and textural information from everything brighter than 1 stop above middle grey, instead of the amount of data doubling for each stop up you go in exposure, it’s held at a constant amount. Normally this is largely transparent as human vision is less acute in the highlight range, but it is still a compromise.
The idea behind Linear raw is that it should give nothing away, each stop SHOULD contain double the data as the one below. But if you only have 12 bit data that would only allow you to record 11 stops of dynamic range as you would quickly run out of code values. So Sony have to use floating point math or something very similar to reduce the size of each stop by diving down the number of code values each stop has. This has almost no impact on highlights where you start off with 100’s or 1000’s values but in the shadows where a stop may only have 8 or 16 values dividing by 4 means you now only have 2 or 4 tonal levels. So once again this is a compromise recording format. To record a big dynamic range using linear what you really need is 16 bit data.
In summary so far:
S-Log reduces the number of highlight tonal values to fit it a big DR in a normal sized bucket.
Sony’s FSRaw, 12 Bit Linear reduces the number of tonal Values across the entire range to fit it in a compact 12 bit recording bucket, but the assumption is that the recording will be at least 12 bit. The greatest impact of the reduction is in the shadows.
Convert 12 bit linear to 10 bit S-Log and now you are compromising both the highlight range and the shadow range. You have the worst of both, you have 10 bit S-Log but with much less shadow data than the S-log straight from the camera. It’s really not a good thing to do and the internally generated S-Log won’t have shadows compromised in the same way.
If you have even the tiniest bit of under exposure or you attempt to lift the shadows in any way this will accentuate the reduced shadow data and banding is highly likely as the values become stretched even further apart as you bring them up the output gamma range.
If you expose brightly and then reduce the shadows this has the effect of compressing the values closer together or pushing them further down the output curve, closing them together as they go down the output gamma range, this reduces banding. This is one of the reasons why exposing more brightly can often help both log and raw recordings. So a bit of over exposure might help, but any under exposure is really, really going to hurt. Again, you would probably be better off using the internally generated S-Log.
To make matters worse there is also often an issue with S-Log in a ProRes file.
If all that is not enough there is also a big problem in the way ProRes files record S-Log. S-Log should always be recorded as full range data. When you record an internal XAVC file the metadata in the clips tells the edit or grading software that the file is full range. Then when you apply a LUT or do your grading the correct transforms occur and all shadow textures are preserved. But ProRes files are by default treated as legal range files. So when you record full range S-Log inside a ProRes file there is a high likelihood that your edit or grading software will handle the data in the clip incorrectly and this too can lead to problems in the shadows including truncated data, clipping and banding, even though the actual recorded data may be OK. This is purely a metadata issue, grading software such as DaVinci resolve can be forced to treat the ProRes files as full range.
This topic comes up a lot. Whenever I have been in discussion with those that should know within Sony they have made it clear that the FS-Raw system is designed around S-Log2 for monitoring and post production etc. This stems from the fact that FS-Raw, the 12 bit linear raw from the FS700, FS7 and FS5 was first developed for the FS700 and that camera only had SGamut and S-Log2. S-Log3 didn’t come until a little later.
The idea is that if the camera is set to SGamut + S-Log2 it is optimised for the best possible performance. The raw signal is then passed to the raw recorder where it will be recorded. For a raw recorder that is going to convert the raw to ProRes or DNxHD the recorder converts the raw to S-SGamut + Log2 so that it will match any internal recordings.
Finally in post the grading software would take the FS-Raw and convert it to SGamut + S-Log2 for further grading. By keeping everything as SGamut and S-Log2 throughout the workflow your brightness levels, the look of the image and any LUT’s that you might use will be the same. Internal and external recordings will look the same. And this has been my experience. Use PP7 with SGamut and S-Log2 and the workflow works as expected.
What about the other Gamuts?
However: The FS5 also has SGamut3, SGamut3.cine and S-Log3 available in the picture profiles. When shooting Log many people prefer S-Log3 and SGamut3.cine. Some people find it easier to grade S-Log3 and there are more LUT’s available for S-Log3/SGamut3.cine than for SGamut and S-Log2. So there are many people that like to use PP8 or PP9 for internal S-Log.
However, switching the FS5’s gamma from S-log2 to S-log3 makes no difference to the raw output. And it won’t make your recorder convert the raw to ProRes/DNxHD as S-Log3 if that’s what you are hoping for. But changing the gamut does have an effect on the colors in the image.
But shouldn’t raw be just raw sensor data?
For me this is interesting, because if the camera is recording the raw sensor output, changing the Gamut shouldn’t really change what’s in the raw recording. So the fact that the image changes when you change the Gamut tells me that the camera is doing some form of processing or gain/gamma adjustment to the signal coming from the sensor. So to try and figure out what is happening and whether you should still always stick to SGamut I decided to do a little bit of testing. The testing was only done on an FS5 so the results are only applicable to the FS5. I can’t recall seeing these same changes with the FS7.
DSC Labs Chroma Tru Test Chart.
For the tests I used a DSC Labs Chroma-tru chart as this allows you to see how the colors and contrast in what you record changes both visually and with a vectorscope/waveform. As well as the chart that you shoot, you download a matching reference overlay file that you can superimpose over the clip in post to visually see any differences between the reference overlay and the way the shot has been captured and decoded. It is also possible to place another small reference chart directly in front of the monitor screen if you need to evaluate the monitor or any other aspects of your full end to end production system. It’s a very clever system and I like it because as well as being able to measure differences with scopes you can also see any differences quite clearly without any sophisticated measuring equipment.
The chart was illuminated with a mix of mostly real daylight and a bit of 5600K daylight balanced light from a Stella LED lamp. I wanted a lot of real daylight to minimise any errors that could creep in from the spectrum of the LED light (The Stellas are very good but you can’t beat real daylight). The camera was set to 2000 ISO. The raw signal was passed from the camera to an Atomos Shogun Inferno where the clips were recorded as both ProRes Raw and also by using the recorders built in conversion to S-Log2 for internal recording as ProRes HQ. I did one pass of correctly exposed clips and a second pass where the clips were under exposed by 1 stop to asses noise levels. The lens was the 18-105mm kit lens, which without the cameras built in lens compensation does show a fair bit of barrel distortion as you will see!
The ProRes clips were evaluated in DaVinci Resolve using the DaVinci Color Managed workflow with the input colorspace set to S-Log2/Sgamut for every clip and output colorspace set to 709. I also had to set the input range of the ProRes clips to Full Range as this is what S-Log2 files always are. If I didn’t change the input range to Full Range the clips exhibited clipped and crushed black after conversion to 709, this confirms that the clips recorded by the Shogun were Full Range – which follows the S-Log specifications.
I did also take a look at the clips in Adobe Premiere and saw very similar results to Resolve.
I will do a separate report on my findings with the ProRes Raw in FCP as soon as I get time to check out the ProRes raw files properly.
So, what did I find?
In the images below the reference file has been overlaid on the very center of the clip. It can be a little hard to see. In a perfect system it would be impossible to see. But you can never capture the full contrast of the chart 1:1 and all cameras exhibit some color response imperfections. But the closer the center overlay is to the captured chart, the more accurate the system is. Note you can click on any of the capture examples to view a larger version.
Below is Picture Profile 6 (PP6) SGamut with S-Log2. It’s pretty good match. The camera didn’t quite capture the full contrast of the chart and that’s to be expected, reflections etc make it very difficult to get perfect blacks and shadow areas. But color wise it looks quite reasonable although the light blue’s are a little weak/pink.
Below is Picture Profile 7 (PP7) SGamut3 with S-Log3. Straight away we can see that even though the camera was set to S-log3, the contrast is the same in the S-Log2 color managed workflow proving that the gamma of the recording is actually ProRes recording from the Shogun is S-log2, confirming what we already know which is that changing the log curve in the camera makes no difference to the raw recording and no difference to the raw to ProRes conversion in the recorder.
Note the extra noise in the greens. The greens appear to have more color, but they also appear a little darker. If you reduce the brightness of a color without altering the saturation the color appears to be deeper and I think that is what is happening here, it is a lightness change rather than just a saturation change. There is also more noise in the darker bars, grey and black really are quite noisy. Light blues have the same weak/pink appearance and there is a distinct green tint to the white, grey and black bars.
Below is when the camera was set to SGamut3.cine with S-Log3. Again we can see that the recording gamma is obviously S-Log2. The greens are still a touch stronger looking but now there is less noise in the greens. Cyan and reds are slightly lighter than SGamut and yellows appear a bit darker. This is also a little more noisy overall than SGamut, but not as bad as SGamut3. When you play the 3 clips, overall SGamut has the least noise, SGamut3.cine is next and then SGamut3 is clearly the noisiest. As with SGamut there is a distinct green tint to the white, grey and black bars.
So that’s what the images look like, what do the scopes tell us. Again I will start with SGamut and we can see that the color response is pretty accurate. This suggests that Atomos do a good job of converting the raw to S-Log2/SGamut before it’s recorded and confirms what we already know which is this is that this is clearly how the system is designed to work. Note how the Red strip falls very close to the R box on the 2x vectorscope, yello almost in Y, green very close to G, Blue almost in B. Magenta isn’t so clever and this probably explains why the pinky blues at the top of the chart are not quite right. Do remember that all these test were done with the preset white balance so it’s not surprising to see some small offsets as the white balance won’t have been absolutely perfect. But that imperfection will be the same across all of my test examples.
Below is SGamut3. The first thing I noticed was all the extra noise on the right side of the waveform where the greens are. The waveform also shows the difference in lightness compared to SGamut with different colors being reproduced at different brightness levels. The greens are being reproduced at a slightly lower luma level and this is probably why the greens appear more saturated. Also notice how much more fuzzy the vectorscope is, this is due to some extra chroma noise. There is a bit more red and magenta is closer to it’s target box, but all the other key colors are further from their boxes. Yellow and Green and Cyan are all a long way from their target boxes. Overall the color is much less accurate than SGamut and there is more chroma noise.
And finally below is SGamut3.cine. There is less noise on the green side of the waveform than SGamut and SGamut3 but we still have a slightly lower luma level for green, making green appear more saturated. Again overall color accuracy is not as good as SGamut. But the vector scope is still quite fuzzy due to chroma noise.
I just want to show you a couple of under exposed examples. These have had the under exposure corrected in post. Below is SGamut and as you can see it is a bit noisy when under exposed. That shouldn’t be a surprise, under expose and you will get noisy pictures.
Below is SGamut3 and you can really see how much noisier this is than SGamut. I recommend clicking on the images to see a full screen version. You will see that as well as the noise in the greens there is more chroma noise in the blacks and greys. There also seems to be a stronger shift towards blue/green in the whites/greys in the under exposed SGamut3.
Clearly changing the gamut makes a difference to the raw output signal. In theory this shouldn’t really happen. Raw is supposed to be the unprocessed sensor output. But these test show that there is a fair bit of processing going on in the FS5 before the raw is output. It’s already known that the white balance is baked in. This is quite easy to do as changing the white balance is largely just a matter of changing the gain on the pixels that represent red and blue relative to green. This can be done before the image is converted to a color image.
What I believe I am seeing in this test is something more complex than that. I’m seeing changes in the luminance and gain levels of different colors relative to each other. So what I suspect is happening is that the camera is making some independent adjustments to the gamma of the Red, Blue and Green pixels before the raw signal is output. This is probably a hang over from adjustments that need to be made when recording S-Log2 and S-Log3 internally rather than something being done to deliberately adjust the raw output. But I didn’t design the camera so I can’t be sure that this is really the case. Only Sony would know the truth.
Does it matter?
Yes and no. If you have been using SGamut3.cine and have been getting the results you want, then, no it doesn’t really matter. I would probably avoid SGamut3. It really is very noisy in the greens and shadows compared to the other two. I would be a little concerned by the green tint in the parts of the image that should be colour free in both SGamut3 and SGamut3.cine. That would make grading a little tougher than it should be.
So my advice remains unchanged and continues to match Sony’s recommendation. This is that you should use PP7 with SGamut and S-Log2 when outputting raw. That doesn’t mean you can’t use the other Gamuts and your milage may vary, but these tests do for me at least confirm my reasons for sticking with PP7.
Both Premiere and Resolve show the same behaviour. Next I want to take a look at what happens in FCP with the ProRes Raw clips. This could prove interesting as FCP decodes and converts the FS-Raw to S-Log3 and SGamut3.cine rather than S-Log2/Sgamut by default. Whether this will make any difference I don’t know. What I do know is that having a recorder that’s converting to S-Log2 for display and software that converts to S-Log3 is very confusing as you need different LUT’s for post and the recorder if you want to use LUT’s for your monitoring. But FCP will have to wait for another day. I have paying work to do first.
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 is a much discussed topic right now, so as I promised in my last article about this, I have put together a video. Unfortunately YouTube’s compression masks many of the differences between the UHD XAVC and the ProRes Raw, but you can still see them, especially on the waveform scopes.
To really appreciate the difference you should watch the video on a large screen at at high quality, preferably 4K.
Camera setup, reviews, tutorials and information for pro camcorder users from Alister Chapman.