Canon F-1 Instructions Manual page 13

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Lens Release Button
Procedure for an FD lens which lacks a chrome
Breech-lock mount ring:
Mounting
1
Align the projecting red mount positioning point with the
red dot above the camera mount.
2
In this position, apply slight pressure to the lens, and
simply rotate the whole lens clockwise until it stops and
the lens release button pops out with a click.
Do not press the lens release button while mounting the
lens. Only when this button pops out can you be sure
that the lens is properly mounted and that it will func-
tion properly. It is also possible to mount this type of
lens when it is not perfectly aligned with this camera.
To facilitate mounting when it is very dark or when you
are in a great hurry, the mount positioning point is
rounded. Simply find this point with your finger and
align it as closely as possible with the red dot on the
camera. Turn the lens slightly back and forth while
applying slight pressure until it drops into position and
continue with step 2 above. Excessive sloppiness will
make mounting impossible; take care to be as accurate as
possible.
Dismounting
1
Turn the lens counterclockwise until it stops while
pressing the lens release button.
2
Pull the lens out.
When the lens is dismounted, the diaphragm blades are
locked in a half-closed position and will not move
even if you turn the lens aperture ring.