Film cameras often are TTL-OTF, Thru-the-Lens Off-the-film. I don't know enough about the design of various EOS film camera metering to comment on how they all do it...I know that SOME models of EOS film SLR (EOS 1n, 5, 10)do have sensors that read the film surface.
Auto exposure with dSLRs face the issue that reflectivity of the sensor surface prevents directly having photosensor pointed at the focal plane, but have to read with sensors up in the pentaprism. And stopped-down metering, in using adapted manual focus lenses on Canon dSLRs, is probably the source of the problem.
I have done extensive testing with a variety of Olympus and Tamron lenses -- both with 'chipped adapters' and 'unchipped adapters' -- adapted to my Canon dSLR, and the most accurate metering is at f/4, but as you stop down (or even open up from f/4) the metering accuracy suffers. A -1EV change in lens aperture does NOT necessarily manifest in a +1EV change in shutter speed! and -- very, very puzzling -- each adapted lens is different from what happens with another adapted lens!
Last edited by wiltw; 05-19-2013 at 06:06 PM. Click to view previous post history.