Shutter Button Lock - Pentax MX Manual

SHUTTER BUTTON LOCK
The shutter button lock lever is for your
convenience while you are making a particularly
long exposure using the B shutter speed setting.
After depressing the shutter button^ turn the lock
lever (which is on the collar at the base of the
shutter button) so that the letter "L" is visible.
When the exposure time is up, move the lock
lever back to its original position. This
automatically releases the shutter button. In
addition to making "Time"' exposure, the lock
lever is also used to turn off the meter circuit and
simultaneously prevent accidental release of the
shutter.
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