Yes. It's a pretty clever circuit. You can think of film as integrating the amount of light that falls on it, so the photodiode integrator is computing the same thing. So the film will get enough light at the same time the integrator gets enough light. By closing the shutter at that time, the film is properly exposed.
The Olympus OM series had some particularly ingenious integrative metering modes, including metering light reflected off the film itself. https://www.dpreview.com/forums/post/24653220
That's very interesting. Just picked up an OM-4 the other week as my first film camera, I knew it was one of the first to have multi-spot metering but just assumed it was standard TTL technology.
Not sure if I'm mis-reading this a surprised reaction, but if so, using light reflected from the film inside the camera body _is_ the standard TTL technology.
Interesting. The parent thread in the linked forum said that other bodies used a different method, so I assumed that was the standard.
> Yep, it applies to almost every AE SLR ever made. Notable exceptions were cameras like the OM-4 that metered off the film during exposure, or others that meter off the closed shutter curtain before exposure. But the vast majority meter from the focus screen so are susceptible to light entering through the eyepiece.
I have an Olympus XA that similarly has a light meter that's active when the shutter's open and closes it when it's "full."
It actually has two light meters, and one of the problems it can have (mine did) is that the light meter that tells you what your exposure will be in the view finder and the light meter inside the body may not agree. The issue was that the internal meter became far slower such that I would see a (correct) value in the viewfinder and then the film would be overexposed by 2 stops. I tested this using slide film and lying to the camera about the ISO by 2 stops and suddenly everything wasn't blown out anymore.
There's a potentiometer hidden inside the body that you can use to adjust the internal sensor's timing (fill rate?) and using audacity to measure the ms between shutter clicks I was able to adjust it to match the viewfinder's reading again.