The term refers to a condition in older cameral lenses, when
oil on the aperture blades congeals over time, causing the
blades to close slowly or not at all. This is a problem because
on a camera the aperture is always wide open until the moment
the shutter is pressed, and then the aperture is supposed
to snap closed to the appropriate setting for the duration
of the exposure, then open up again. If the blades close only
slowly, then too much light will reach the negative, and the
picture will be ruined. A lens with a sticky aperture is essentially
useless for regular photography, and you should check any
used lens you are considering for this condition before purchase.
Lenses can be tested for this both on and off a camera. On
a Minolta Auto Focus or similar camera, set the aperture manually
to f/22 and then press the Depth Of Field (DOF) Preview button.
With a properly functioning lens, the viewfinder will become
instantly darker. With a lens with a sticky aperture, the
viewfinder will become gradually darker. If the camera has
no DOF preview button, open the back of the camera and look
through it, set the shutter to B and hold the shutter release
down. You will see through the lens what the aperture blades
are doing. Off the camera, you can test a lens by removing
both caps and looking through the lens. The aperture will
be wide open. On the rear of the lens in the mount area there
is a bar for the camera to move the aperture. With a fingernail
you can slide the aperture closed, and watch it close through
the lens. You will be able to clearly see the aperture blades.
Having closed the aperture all the way, you can let go. On
a normal lens, the aperture will snap back open. If the aperture
is sticky, it will gradually open, or, if it is really bad,
not open at all.
Fixing a sticky aperture requires disassembling the lens
and cleaning the aperture blades with a degreaser, then oiling
them again. This costs between $60 and $150, and is not something
one can attempt at home.
Some discussions of sticky apertures that I have been able
to find on the web:
http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=000isF
http://www.acecam.com/message/4455.html