Canon manual 79

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Aperture Signal	AE Switch Pin
Lever	Automatic
Full Aperture
	Aperture Lever
Signal Pin

Reserved Pin

Lens Signal Coupling

There are several levers and pins at the rear of an FD lens which transmit signals between the lens and the camera body. Usually it is not necessary to know what they are, but sometimes, when you must operate the aperture manually for non-coupled accessories for instance, it is. Just for your information, we include explanations of them here.

1. Aperture Signal Lever

With an FD lens, exposure metering is performed through the lens at full aperture. This is nice for you since you have a bright viewfinder to focus and compose, but to determine correct exposure, the camera must know the effective aperture. For most cameras, such as the Canon F-1, this lever transmits the preset aperture on the lens aperture ring to the exposure meter. It is coupled to the lens aperture ring and moves in proportion to its rotation. In AE photography, however, the exposure meter receives a signal directly from the camera.

2. Automatic Aperture Lever

This lever couples to the camera body to stop the diaphragm down to the preset aperture just before the shutter releases. This is the lever that must be locked for manual diaphragm control with a non-coupled acces- sory.

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