Canon AT 1 - Manual (Page 33)

Shutter Button and Shutter Lock
The magnetic release shutter button
enables smoother shutter release than the
mechanical release method does. There is also
less chance for camera shake.
When the shutter lock lever around the
shutter release button is turned to the "L"
position, the shutter button will be locked to
prevent unintentional shutter release. Keep
the shutter release button locked while
carrying the camera to prevent film waste.
When the power level of the battery is
insufficient, a safety mechanism will keep the
shutter from being released.
■ At temperatures under —20 degrees C,
there may be an occurrence when the shutter
will not be released depending on the
batteries, even if the battery power is suf­
ficient. In that cold condition, the battery's
power is reduced so some 10 seconds should
be allowed after the battery is checked before
taking a picture.