exposing FX6 (CINE EI/Slog3) + A7S3 (Slog3)+ A74 (Slog3) to match

[Jason OHara]

I am setting up a three camera shoot, and need to match the images from all three cameras to inter-cut.
I typically film with FX6, in CINE EI mode, and follow Alister Chapman's exposure advice using a grey card - exposed at IRE=41%:
https://sonycine.com/articles/how-correctly-expose-s-log3---a7s-iii---fx3---fx6---fx9/

I understand the grey card should also be exposed at 41% for the A7S3 & A74 for Slog3, however I assume that when using CINE EI on the FX6, I am in essence "over exposing" by one stop (at CINE EI 400) or by two-stops (at CINE EI 200), and so presumably, I should be compensating for this on both the A7S3 & A74?

And so if my understanding is correct (and if not, please correct me 🙂) what should the IRE value be when exposing the grey card on the A7S3/A74* to match the FX6 fiming in CINE EI at the FX6 base ISO 800?  (*and locking the ISO to the base ISO=800 on A74, and base ISO=640 on A7S3). I read online that 1 stop corresponds to 10% IRE, so would I simply add 10% to 41% for every stop of desired "over-exposure" on the A7S3/A74, to match the impact of CINE EI on the FX6? i.e.

grey card @ 51% on A7S3(iso640)/A74(iso800)? to match FX6’s grey card =41%, when filming CINE EI 400 (thus “over-exposing” the Slog3 on A7S3/A74 by 1 stop)
grey card @ 61%on A7S3(iso640)/A74(iso800)? to match FX6’s grey card =41%, when filming CINE EI 200 (thus “over-exposing” the Slog3 on A7S3/A74 by 2 stops)

I appreciate clarification on the above, and ANY other advice/tips regarding my intention to mix footage from these 3 cameras.
Thank you!

 

[Nicolas POISSON]

I guess you want to adjust the exposure using the aperture ? A very simple way would be to set your A7s so that the grey card falls at 41 %, then open the aperture one or two stops.

If you prefer maths, you need the Slog3 Formula :

  out = (420 + log10( (in + 0.01) / 0.19) * 261.5) / 1023

• ‘out’ is the target IRE value
• ‘in’ is the reflectance. For middle grey, IN = 0.18 (18%). One stop above is IN = 0.36, two stops is IN = 0.72

Target IRE is 41 % (0.41) for middle grey, 48 % one stop above, 56 % two stops above.

The ‘10 % per stop’ rule is very generic. It depends on the log type, and even for a specified log, it depends on were you are on the curve. In the linear section, we get close to 7.5 % per stop in Slog3.

Edited February 11, 2025 by Nicolas POISSON