Canon A-1 - Manual (Page 51)

Magnifier S
The Magnifier S inserts into the grooves of
the viewfinder eyepiece with its adapter to
give a 2.5X magnification of the center of
the viewing area for precision focusing in
close-up work and wide-angle photography.
Its power is adjustable to your eyesight
within a range of +4 to -4 diopters. Its
adapter is hinged so that the magnifier can
be swung upward from the eyepiece after
focusing, leaving the entire screen image
visible.
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