Exposure difference between Sony a7III and Portra 400

Luca - Italy - Sony A7III - Canon 1V - Fashion

Hello everyone,

I usually use the Sony as the light meter for my film shoots. I make always a test with the Sony at 400 iso and then shoot same on film camera with Portra 400. I was thinking how many stops of difference there are between the Sony A7III and the Portra 400 at same setting (exp and shutter) on a Canon 1V with same flash light (Profoto b10 with a snoot)

This is because on my next shooting, I want to use a very famous and simple technique:

Exposing the sky behind the subject (model) and light up the model with a shaped flash.

I did it sometimes, but I'm always afraid of the difference of exposure and contrast between the cameras, especially with flashlight.

If you see the example below, there is a minimum of backlight behind the head of the model. I'm sure that if I see that imperceptible backlight on the Sony A7III, I will never see it on film, with same camera settings. Am I wrong?

THANK YOU EVERYONE

Here's the example from photographer Dan Beleiu:

I'd think you'd want to test. While I'd expect an external lighting set up to be camera agnostic, I remember people buying film out of the same lots for consistency and "rumors" that some digitals were maybe optimistic in discussing performance at different isos (and comparative testing, etc.).

I might expect the results to be close but not necessarily the same. The cameras are different, as would be lenses (probably), there's darkroom and digital post going on/available, too.