Canon AT 1 - Manual (Page 54)

Dioptric Adjustment Lens S
Dioptric adjustment lenses can be
attached by inserting them from above into'
the grooves in the viewfinder eyepiece to
compensate for the individual eyesight. With
them, near-sighted or far-sighted persons can
perform photography without glasses.
The built-in eyepiece lens of the AT-1 has
—1 diopter. The following 10 kinds of
dioptric adjustment lenses are optional acces­
sories: -1-3,
+ 2 ,
-M.5, -1-1, -1-0.5, 0, —0.5, —2,
—3 and —4 (diopters).
One way of selecting the correct dioptric
adjustment lens for you is to select the one
that is the closest to your glasses in regard to
number of diopters. But, we propose that,
to select the most appropriate dioptric ad­
justment lens, you actually look through
the viewfinder after placing it over the eye­
piece.
Because the camera itself has —1 diopter,
the diopters of the lenses are recorded as the
real power when attached to the camera, thus
reflecting the power of the camera's viewfin­
der.