Canon F1 Quick Start Manual page 3

When the mark on the lock ring is not turned to the "A" position, it is impossible to
proceed to press the shutter release button or to wind film. This device is useful
when carrying the camera around. Shutter release button lock should be turned to-
the "L" position to avoid accidental tripping of the shutter.
Viewfinder
A standard ring with -1.2 diopters is attached to the eyepiece. Those who cannot
see through it clearly can interchange it with any of the other dioptric adjustment
lenses. The focusing screen inside the viewfinder box can also be interchanged. A
focusing screen A of the standard type is installed at the factory.
The following information will be obtained through the viewfinder.
Focusing