Understanding Sony’s SLog3. It isn’t really noisy.

It’s been brought to my attention that there is a lot of concern about the apparent noise levels when using Sony’s new Slog3 gamma curve. The problem being that when you view the ungraded Slog3 it appears to have more noise in the shadows than Slog2. Many are concerned that this “extra” noise will end up making the final pictures nosier. The reality is that this is not the case, you won’t get any extra noise using Slog3 over Slog2 and Slog3 is generally easier to grade and work with in post production.

So what’s going on?

Slog3 mimics the Cineon Log curve. As a result the shadow and low key parts of the scene are shown and recorded at a brighter level than Slog2. Because the shadows are brighter, the noise in the shadows appears to be worse. It isn’t. The noise level might be a bit higher but the important thing, the ratio between wanted picture information and un wanted noise is exactly the same whether in Slog2 or Slog3 (or in fact any other of the cameras gamma curves at the native ISO).

Let me explain:

The signal to noise ratio of a camera is determined almost entirely by the sensor. This is NOT changing between gamma curves.

The other thing that effects the signal to noise ratio is the exposure level, or to be more precise the aperture and how much light falls on the sensor. This should be same for Slog2 and Slog3. So again no change there.

As these two key factors do not change when you switch between Slog2 and slog3, there is no change in the signal to noise ratio between Slog2 and Slog3. It is the ratio between wanted picture information and noise that is important. Not the noise level, but the ratio. What people see when they look at ungraded SLog3 is a higher noise level (because the signal levels are also higher), but the ratio between the wanted signal and the noise is actually no different for both Slog2 and Slog3 and it’s that ratio that will determine how noisy your pictures will be after grading.

Gamma is just gain, nothing more, nothing less, just applied by variable amounts at different levels. In the case of log, the amount of gain decreases as you go further up the curve.

Increasing or decreasing gain does NOT significantly change the signal to noise ratio of a digital camera (or any other digital system). It might make noise more visible if you are amplifying the noise more, for example in an under exposure situation where you add gain to make a very dark object brighter. But the ratio between the dark object and the noise does not change, it’s just that as you have made the dark object brighter by adding gain, you have also made the noise brighter by the same amount, so the noise also becomes brighter and thus more obvious. The ratio between the wanted signal and the unwanted noise remains constant, no matter what the gain, it is a ratio and gain does not change ratios. With Slog3 in post production you will need less gain in the shadows than you would with Slog2 and this negates the extra gain that the camera adds to the shadows when shooting SLog3.

Lets take a look at some Math. I’ll keep it very simple, I promise!

Just for a moment to keep things simple, lets say some camera has a signal to noise ratio of 3:1 (SNR is normally measured in db, but I’m going to keep things really simple here).

So, from the sensor if my picture signal is 3 then my noise will be 1, or if my picture signal is 6 then my noise will be 2.

If I apply Gamma Curve “A” which has 2x gain then my picture becomes (6×2) 12 and my noise (2×2) 4. The SNR is 12:4 = 3:1

If I apply Gamma Curve “B” which has 3x gain then my picture becomes (6×3) 18 and my noise becomes (3×2) 6. The SNR is 18:6 = 3:1 so no change to the ratio, but the noise is 6 compared to the 4 of Gamma “A”, as a result Gamma “B” will appear to be noisier when viewed on a monitor.

Now we take those imaginary clips in to post production:

In post we want to grade the shots so that we end up with the same brightness of image, so lets say our target level after grading is 15.

For the gamma “A” signal we need to add 1.25x gain to take 12 to 15. As a result the noise now becomes (1.25 x 4) 5.

For the gamma “B” signal (our noisy looking one) we need to use 0.8333x gain to take 18  to 15. As a result the noise now becomes (0.83333 x 6) 5.

Notice anything? In both cases the noise in the final image is exactly the same.

OK, so that’s the theory, what about in practice?

Take a look at the images below. These are 400% crops from larger frames. Identical exposure, workflow and processing for each. You will see the original Slog2 and SLog3 plus the Slog 2 and Slog 3 after applying the LC-709 LUT to each in Sony’s raw viewer. Nothing else has been done to the clips. You can “see” more noise in the raised shadows in the untouched SLog3, but after applying the LUTs the noise levels are the same. This is because the Signal to Noise ratio of both curves is the same and after adding the LUT’s the total gain applied (camera gain + LUT gain) to get the same output levels is the same.

Slog2-400
Slog3-400Slog2-to-709-400Slog3-to-709-400

It’s interesting to note in these frame grabs that you can actually see the improvement in shadow detail that SLog3 brings. The bobbles and the edge of the picture frame look better in the Slog3 in my opinion. A little bit more shadow data has given a more pleasing result with fewer artefacts.

The only way you can alter the SNR of the system (other than through electronic noise reduction) is by changing the exposure, which is why EI is so important and so effective.

Noise is always most problematic in shadows and low key. As we are putting more data into the shadows with SLog3 we are in effect recording the noise in the shadows more precisely, you won’t enhance it or increase it. All that will happen is that it is more accurately reproduced with fewer artefacts, which is a good thing.

In addition Slog3 has a near straight line curve. This means that in post production it’s easier to grade as adjustments to one part of the image will have a similar effect to other parts of the image. It’s also very, very close to Cineon and to Arri Log C and in many cases LUT and grades designed for these gammas will also work pretty well with SLog3.

The down side to Slog3?

Very few really. Fewer data points are recorded for each stop in the brighter parts of the picture and highlights compared to Slog2. As a result Slog3 is slightly less forgiving of overexposure than Slog2. You probably don’t want to push your EI gain quite as hard with Slog3. 1.5 stops over should be OK (so using an EI 1.5 stops down from native) but 2 or more will hurt your pictures.

Want to pick my brain for 10 days, fancy an adventure and a chance to see and shoot some very cool sights? Why not join me for a storm chasing adventure. 

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6 Responses to Understanding Sony’s SLog3. It isn’t really noisy.

  1. Pingback: Understanding Sony’s SLog3. It isn’t really noisy. | roberto cimatti

  2. Pingback: Understanding Sony’s SLog3. It isn’t really noisy. | London Cameraman

  3. Brook Aitken says:

    Is the last paragraph you say you may not want to push the EL gain quite as hard with S-log 3, did you mean in post? Right?
    Thank you in advance for all your info and advice we all really appreciate it!

    Brook Aitken
    DP
    http://www.brookaitken.com

    • alisterchapman says:

      Raw is linear, so where you place your exposure within the cameras 14 stop range makes little difference (other than noise). S-Log2 and S-Log3 use log encoding so as you go up the exposure range less data is recorded relative to the amount of information in the scene. Over exposing log by more than 1.5 stops (Using an EI 1.5 stops down) can make it harder to grade the image as a result.

  4. Krishna devate says:

    How do I expose for slog 3 or slog 2 with a7s ii.

    Two and a third stop more or 3+ stops more

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