Canon AE-1 PROGRAM Instructions Manual page 50

Notes
Depth of field is also greater the shorter the focal
length of the lens. For example, a 24 mm lens will
show greater depth of field than a 50 mm lens, pro-
vided the aperture and shooting distance are the
same. Depth of field is also greater the longer the
shooting distance, and is generally greater in the
background than in the foreground by a ratio of two
to one.
With a Canon FD lens, viewing and metering are
done at maximum aperture where the viewfinder is
brightest. The lens diaphragm does not close to the
shooting aperture until the shutter is released .
Afterwards, it reopens automatically to the max-
imum aperture. Because the maximum aperture pro-
vides the narrowest range of sharpness, the subject
is viewed with the shallowest depth of field .
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