Canon New F-1 Instruction Manual page 53

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24 DEPTH OF FIELD
You can check depth of field visually in the following way:
Focus the subject. Then press the shutter button halfway and note to hich
f/stop the meter needle points in the viewfinder.
Turn the lens' aperture ring to that f/stop.
Push in and then release the stop-down slide so that it unlocks and you can
see its red line. Now, just by looking at your subject through the viewfinder,
you can see the range of sharp focus.
After checking depth of field, push in and then release the stop-down slide.
Note
It is impossible to release the stop-down slide when the lens' aperture ring is on
"A." If you are using the camera in shutter-priority AK, first disengage the lens
from "A" and set the working aperture. After checking depth of field, return the
aperture ring to the "A" mark.
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