Canon AE-1 Manual page 23

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Confirming the Depth-of-Field by: 1). Stopping-Down the Lens Diaphragm. Wind the film

and determine the aperture required for the subject by metering, and then set the f/stop by

turning the aperture ring after disengaging the aperture ring from the "A" mark. 2). Press the

stopped-down lever until it locks. Once locked, the depth-of-field can be checked by looking

into the viewfinder. Thus, the extent of the depth-of-field can be seen as the zone of sharpness

in the subject field observed on the screen. When the stoppeddown lever's release button is

pressed, full aperture metering will be restored.

Stopping-down of the FD lenses should only be done after advancing the film. Should you

not advance the film, the stopping-down of the lens diaphragm would only be possible

up to the previous exposures aperture. Also, when the aperture ring is set at the "A"

mark, the stopped-down lever cannot be pressed.

Generally, the depth-of-field will become deeper as the aperture becomes smaller, and

shallower as the aperture becomes larger. A shorter focal length as well as a greater subject

distance will also attain deeper zone of depth-of-field. Comparing an interchangeable 28mm

lens with a standard 50mm lens set at the same f/stop, the 28mm lens's depth-of-field will be

greater. And when the photographic distance changes, the depth-of-field changes, too. For

example, if the same subject is photographed from three and then from seven meters away, the

foreground and background of the subject will be deeper at the greater distance.

4) Depth-of-Field Scale on the Lens Most manual focus lenses has excellent depth-of-field

scale engraved on the lens barrel, shown as a series of f/numbers on each side of the distance

index mark opposite the distance scale. Focusing and depth-of-field are so closely interrelated

that the depth of-field scale is engraved together with the distance scale. You can tell the

extent of depth-of-field from the distance scale.

There are two methods of confirming the extent of the depth of the

field: by stopping down the lens diaphragm or by reading a value from

the depth-of-field scale on the lens.

3). After having turned the aperture ring to the maximum f/stop on the

lens, reset it to the "A" mark. Otherwise, you will not be able to obtain

the correct automatic exposure for the next shot.