What is a sticky aperture?

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