SLOG3 and ISO 800

Started 3 months ago | Discussions thread
Dave_J_E Regular Member • Posts: 139
Re: SLOG3 and ISO 800

MaxN wrote:

I'm curious to understand how my camera works behind the scenes when using SLOG3. I've read some explanations and watched youtube but haven't quite got it yet. I hope someone can help:

My Sony base ISO for SLOG3 is 800. Is this because the SLOG3 gamma pushes the data to the top end such that an 18% grey level is now, what, three stops higher? And so Sony sets the ISO to 800 to trick the metering and put neutral grey objects back down at 18% where they're supposed to be. Is it something like that?

However, then you've wasted a lot of headroom, so normal advice is to expose above indicated exposure by about 1.7 stops (lets say 2 for a round number) to lift exposure above the noise floor. That then effectively gets you back to an exposure equivalent to ISO 200, right?

If so, then why didn't Sony just let me set ISO 200 (or perhaps even 100) when using SLOG3 and save me having to twiddle extra knobs?

As I see it, log profiles are used to capture brighter highlights without clipping and to do this, exposure settings such as aperture and shutter speed must be used to reduce the exposure. This is where the "fake ISO setting" of 800 comes in, to trick the camera's auto exposure meter. There is a downside to this of course and this is lower signal to noise ratio in the shadows. The log curve is effectively a form of compression which allows the use of a 10bit video file whilst minimising the chance of visible banding. If the data is captured at say 14 bits, and then a log curve applied before downsizing to 10 bits, the number of quantisation levels for the highlights are reduced whilst the number of quantisation steps in the shadows remain higher. Perceptually, quantisation steps of a given size are less noticeable in the highlights.

I surmise that the Slog3 profile is designed to cover a certain range of highlights. However in practise the range to be captured will vary according to the scene. I would therefore suggest that the ISO800 figure is used to produce a nominal exposure only and you should vary the exposure to suit the scene you are capturing. But I have no direct experience with this as I don't have a camera that can do video with log profiles. To get best noise performance, you should expose so that the brightest highlights are lust below clipping.

Dave

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