Hi All,
I’m the Ben that Nick mentioned. I don’t have much useful to add, but my understanding of the linear IRE thing is down to the conventional reference white in the real world being a 90% reflectance card . The knee in conventional cameras tended to make 90% reflectance = 90% recording, but without the knee activated 90% reflectance would notionally be 100% recording at nominal exposure, and with conventional gamma 100% linear → 100% gamma corrected.
Now we have cameras capable of huge dynamic ranges and curves deliberately intended to have lots of headroom so that camera midtone exposures can be consistent even when highlights are challenging. We also have ACES designed to bring disparate cameras into a consistent frame of reference, and to get to that frame of reference and keep the maths sensible a well (sensibly) defined scene linear is the way to go, and 90% reflectance is, like 18% grey, a reference point rather than a limit.
I have no idea why Sony (and if I remember correctly Canon with the first C-Log white paper) went with ‘linear IRE’ in their log technical documentation, but I can say that in writing my app it was a headache double checking when the 0.9x factor is needed.
As a fun side experiment, if you have LUTCalc, you can try comparing DJI D-Log gamma to Sony S-Log3 with ‘slope’ set to 0.9 in the ASC-CDL customisation. This is the same as multiplying by 0.9 in linear space (ie 18/20). I got the parameters for both of those curves from the respective manufacturers published documentation,
Ben