Canon AV-1 Lens Mount and Lenses

The Canon AV-1 has a breech-lock mount and can accept FD and FL lenses. The FD lens provides full-aperture metering, because the mechanical lever at the back of the lens transmits the aperture setting to the camera. This way, the meter is able to provide the correct information in the viewfinder, even though the aperture remains fully open. However, with FL lenses, full aperture metering is not possible.

Here is an FD lens showing the mechanical levers.

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