I have a Canon AE-1 film camera. It works well and I've several FD, FL and Vivitar lenses.
For recent digital cameras, I know how the light meter works. However, with this old camera, I would shed light on one specific point.
For full-aperture metering, the meter does the measurement with light rays that come through the lens. There are 4 factors:
- ISO/ASA (known by the camera)
- speed (also known)
- quantity of light (what is actually measured)
- aperture (not known by the film camera ...)
You see where I'm going here. In full-aperture metering, aperture is a factor that depends on the lens and can vary ! I've some 1.4, 1.8, 2.8 and 3.5 lenses. How the light meter can do a correct computation without knowing the aperture of the lens ?
For example, on a sunny day with 100 ASA film, 1/1000 speed and my f/1.4 lens, the light meter of my AE-1 will indicate f/5.6. But if I take another lens, e.g. a f/2.8 one, the quantity of light receive by the light meter will be less than with the previous one... Thus, it will indicate may be f/4 or f/2.8, or some other value. And one of the readings must be wrong, no?