How do analog photography light meters work?

Question

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?

Answer 1

This depends on the lens mount. The Canon FD mount has two mechanical communication paths from the lens to the camera:

• An Aperture Signal Lever that tells the camera whether the lens is at full aperture or, if not, how much it is stepped down.
• A Full Aperture Signal Pin that tells the camera what its full aperture actually is - so the camera does know! When connected, it depresses a matching pin in the camera body. In variable-aperture zoom lenses, working the zoom moves this pin.

The details are explained on this page.

Note that FL mount lenses to not have the Full Aperture Signal Pin, so with such a lens, your meter actually shouldn't be able to tell you what aperture to set, only how much to stop down (or up).