Noise, ISO, Gain, S-Log2 v S-Log3 and exposure.

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.

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12 thoughts on “Noise, ISO, Gain, S-Log2 v S-Log3 and exposure.”

  1. Every time you write about it Alistair, I learn more. Heading to color correct for my film Becoming Jamie: Shock & Joy on Tuesday. Looking forward to seeing how it fares – shot Slog2 over exposed. On my edit screen it’s looked noisy… many thanks

  2. I am using a Sony A7riii . I am shooting in slog2. 24fps and my shutter speed is 50. I then adjust aperture and iso to get my desired look and exposure. I have my zebras set to 70. I expose for people by setting my exposure values just below seeing the zebras on their face. Is this the best way to get proper exposure for their face?

    1. If you are changing the ISO you may be significantly reducing the dynamic range which defeats the object of using S-Log. On the A7R3 you should be sticking to 800 ISO. Raising the ISO decreases the dynamic range limiting the highlights that you can capture and making the image noisy.

      The base exposure for skintones and S-log2 is around 46% and the base brightness for white is 59%. So if you are exposing skin tones at close to 70% you can see that this is actually very over exposed as skin tones fall between middle grey (which is specified as 32% for S-Log2) and white (59%).

      However exposing a bit brighter is typically beneficial as after grading you will have less noise. I find that the Alpha cameras seem to work best when exposed between around 1.5 to 2 stops bright. If you expose at +1.5 stops white will be 70% and at +2 stops white will be 80%. So zebras at 70% just appearing on skin tones means you are around 2 stops brighter than the base exposure, which isn’t a bad place to be as long as you are aware that that is how you are exposing. To deal with this nicely in post production you won’t be able to use normal LUT’s as everything will look over exposed unless you correct the exposure before applying the LUT. Instead you should use exposure corrected LUTs or proper color grading software.

      1. So are you saying when exposing for skin I should have my zebras appear at around 46 to get proper exposure in slog2 and to work better with luts? Or is it better to use a grey card next to talents face and expose for that but what number should I use for the zebras with the grey card? Thank you for all of your help!

  3. So are you saying when exposing for skin I should have my zebras appear at around 46 to get proper exposure in slog2 and to work better with luts? Or is it better to use a grey card next to talents face and expose for that but what number should I use for the zebras with the grey card? Thank you for all of your help!

    1. 46% would be the design brightness for skin tones. S-Log2 middle grey is 32% and white is 59%. Skin tones sit roughly half way between white and middle grey which is 46%

      But S-Log2 was originally designed for a high end 10 bit cameras. When you sue it on an 8 bit camera like the A7 series then most people will find they get a better result by exposing brighter than this and then using a LUT designed for the brighter exposure in post. Exposing skin tones close to 70% is a pretty big amount of over exposure, but it will probably work. I myself would normally aim to have white such as a white card or white piece of paper at 70% and skin tones around 60%.

        1. If the light is poor why are you using log? Log is a very poor performer if you can’t expose it properly. Add gain and you no longer have the high dynamic range benefit that log has and as with any underexposed log all your data will be in the shadow roll off at the bottom of the curve. Either way it isn’t going to look great.

  4. Hi Alister.

    Is there a resource that shows how data is allocated for s-log2 and s-log3? I’ve run into people that believe shooting “flat” (adjustments in matrix) is the same as shooting log. I believe data allocation also plays a big part. I just need to point them in the right direction.

    Cheers.!

  5. is it possible ISO is confusing because those who shoot digital possibly never shot film and so don’t understood ASA – how could they? emulsion is a beast unto its self. add to that a camera operator wasn’t the editor and colourist all built into one person.

    shutter, aperture and asa – the corner stones to understanding the principals of shooting with film. now every camera’s sensor is essentially a roll of film with particular characteristics and sensitivity to light.

    i see you take so much time explaining ISO and its relationship to various shooting conditions as well as by camera and log vs luts vs … and still it never seems to sink in. throw in EI, cine and then post production techniques and it’s like the generation of photographers who try to fix everything in photoshop for all the same reasons users of digital cine cameras have no idea why they should be using a light meter.

    1. The difference is that the ASA/ISO of film is an actual measurement of the density of the film when exposed to a certain amount of light. In other words it is the actual sensitivity of the film. And in a film camera you can change the film you are using to suit the light levels you are working with. Besides this most movie stock would have both an ISO rating (measured sensitivity) and an EI rating (recommended exposure level). EI is nothing new.

      But we can’t change the sensor in our video cameras so we can’t actually change how sensitive the camera is. All we can do is change the gain or amplification, which is very, very different to changing sensitivity. Yet our cameras have many different ISO ratings – this is actually impossible! But it is “possible” because there is no standardised way to rate an electronic camera using ISO, so the camera manufacturers make it up as they go along. Then because it’s much cooler to talk “ISO” than dB of gain users of electronic cameras apply the wrong terms and end up completely confused and lost.

      If you have an electronic camera then there is no reason why you must use a light meter. The camera is the best light meter you’ll ever own.

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