Log curves and dynamic range of shoguns small HD

[Abdul Rahman Jamous]

Good day to you awesome people and hope that you are doing GREAT!

 

I was reading the user manual of Atomos Shogun7 and I came cross this graph. I roughly get what IRE means, but I want to understand what does "Reflectance (%)" mean

[Nicolas POISSON]

Imagine a mate white board that reflects a certain amount of the light it receives, sending it to your camera. You define this quantity to be 100%. You set the exposure of your SDR camera with pure Rec709 profile so that this white board reaches 100% IRE. Now changing the camera  profile, it may be able to capture objects sending 4 times more light (400%), or 15 times more light (1500%) without clipping. In the real world, you may simulate this keeping the same board and just increasing the light source.

This is a bit like estimating the variation of the cost of life through the years: you take the price of various products in 1990 and compute the total amount to be $1200. You define this as your 100% reference. Ten years later, the same products cost $1800. This represents 150% compared to 1990. The absolute values of $1200 and $1800 do not matter, only the ratio is of interest.

Edited July 9, 2023 by Nicolas POISSON

[David Mullen ASC]

Math is not my strong suit, but if 100% is max in SDR, and each f-stop is a doubling of light, then:

200% 1- stop

400% 2-stops

800% 3-stops

1600% 4-stops

Which suggests that Log-C on an ARRI is holding maybe 5-stops more information than a camera set to normal Rec.709 recording (I assume with no unusual knee compression.) Not sure if that is correct, as I suggest, it must depend on how the Rec.709 LUT is set-up.

It does show that you need every bit of dynamic range that a log or raw recording can contain for HDR.

[Nicolas POISSON]

I did not investigate every single camera released recently, but I guess none of them has a strict Rec709 profile nowadays, as it would limit the DR to slightly more than 5 stops (if I understand correctly). I guess all current so-called rec709 profiles include some kind of "S-shape" tone mapping, or at least some highlight roll-off, because nobody would accept only 5 stops of DR, except for a specific project wanting to mimic vintage found footage style. Thus, the diagram in the first post exaggerates the benefit of log curves over SDR: In a 'IRE vs. stop" scale it would appear less impressive, and the "SDR" used as a reference is a norm nobody follows any longer.

I went through the Shogun manual,  and I am a bit confused with that chapter. It seems that this diagram has nothing to do with the way the shogun handles log signals or is able to turn into an HDR display. It is simply a representation of various vendors log curves, nothing more.

Edited July 9, 2023 by Nicolas POISSON