Asker Mammadov Posted July 1, 2022 Share Posted July 1, 2022 This question has already been asked once but that thread was 7 years ago, and since then I'm sure much more information has become available to the public and thus better explanations have also become possible. I'd like to take the example of ARRI's image processing pipeline. At a native ISO of 800 there are 7.8 stops above 18% diffuse reflectance available. Therefore, 18% x 2^(7.8) = 4011% diffuse reflectance is the clipping point. As far as my understanding goes, all cinema cameras sensors (but let's stick with Arri) record light linearly and in turn linearly convert the analog signal to 16bit digital values (Linear ADC) with the voltage generated being proportional to the the digital value(I.e Gamma =1) and only then does the conversion from 16bit Linear values to 12bit Arri LogC take place. Let's also assume all of the 16bit data is available for use, so excluding noise and other limiting factors,in order to make life simpler Now let's say I want to expose for an 18% gray card so that it ends up as middle gray on Arri 12bit LogC( I can't remember exactly what IRE they recommended, but let's assume it was 40% IRE). This is where I start getting confused. On a Linear ADC (assuming we expose correctly LINEARLY so that 18% gray card is 0.4486% LINEAR IRE. Because 18/4011 is 0.4486%) : 18%/4011% x 65535= 294 This means that there are only 294 tones from pitch black to 18% gray card on the original data the linear sensor/ADC recorded. Yet, once the 18% gray card gets converted from Linear 0.4486% IRE to Log 40% IRE on 12bit there are 1638 tones from pitch black to 18% gray card. Where are all these extra tones coming from? Is there interpolation happening here? The whole point of Log recording is to allocate more bit values to the darker areas up till 18% gray where we are more sensitive. Yet, the logarithmic values are all just redistributions of the Linear 16bit values if I understand correctly. Thus, there was never more than 294 tones of true information recorded between pitch black and 18% gray. The only other thing I can think of is that when we're exposing for 18% gray card to be 40% IRE on 12bit LogC we're actually OVEREXPOSING on the Linear Readings so that an 18% gray card is read as 1638/65535 x 4011% = 100% diffuse reflectance(or 2.5% LINEAR IRE). Which gives us 1638 tones of true information because 100%/4011% x 65535=1638 on the 16bit linear values I hope I explained my conundrum clearly. If not, feel free to ask I'll try and explain again. Thanks PS: I know this doesn't help in any way with being a great cinematographer. I just got curious haha. Quote Link to comment Share on other sites More sharing options...
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