“Sticky aperture” refers to a problem where the aperture of a lens closes down too slowly, or not at all. This problem may cause overexposure.
All modern SLR and DSLR lenses comes with what used to be called “auto aperture” (but this feature is now so universal that nobody calls it anything any more). It works like this: Metering and focusing is done with the aperture wide open. Then, at the moment the shutter button is pressed, the aperture snap closed to the aperture set for the exposure. Then it opens up again. The video below show a slow-motion (5000 fps) video recording of a Nikon D3 firing at 11 fps. The exposure aperture is set to f/16 and the video shows how the aperture (on the left) closes down and opens again for each of the frames. The shutter, seen on the left, only opens after the aperture has closed down to the correct f-stop.
The most common cause for this problem is oil on the aperture blades. Oil congeals over time and become “sticky”. This may prevent the aperture blades from closing down correctly during exposure, so that the correct aperture is never reached before the shutter opens. The visible result from this problem is overexposure when the lens is stopped down in any of the auto exposure programs.
The cause may also be with the mechanical lever that some cameras use to stop down the aperture. If this lever is bent or shifted out of position, the lens may not stop down properly.
On camera systems that uses electric signals to control aperture, such as Canon EOS (Electro-Optical System), the problem may also be caused by poor contact between body and lens.
A lens with a sticky aperture is pretty useless. It will only perform as expected at its widest aperture. When you are buying a used lens, a sticky aperture is one of the first problems to check for.
For a quick test, you can check the aperture blades of a lens by using the camera's Depth Of Field (DOF) preview button. Set the lens to its smallest aperture. Look through the viewfinder and press the DOF preview button. With a properly functioning lens, the viewfinder will instantly become much darker. With a lens with a sticky aperture, the viewfinder will become gradually darker, or it will not change at all. Unfortunately, not all camera's have a DOF preview button. If you do not know whether your camera has this feature, look it up in the camera's manual.
On a lens that has a mechanical aperture control lever, such as a Nikon F-mount lens, you can also check the behaviour of the lens' aperture by taking the lens off-camera and removing the caps at both ends. If the lens is not a Nikon G-type lens, make sure the mechanical aperture ring is set to the lens' smallest aperture. Look through the lens from the rear and carefully operate the lever with your index finger. When a Nikon lens is off-camera, it should be closed down to its smallest aperture. When you carefully pull this lever counterclockwise, you should see the lens open up to its largest aperture. When letting the lever go, the aperture should instantly snap shut to the lens' smallest aperture again. If the blades do not move when you do this, or they move slowly, the lens has the sticky aperture problem.
(If the lens has a mechanical aperture lever or pin, but is not a Nikon F-mount lens, the mechanical coupling to the aperture may operate different from what is described in the previous paragraph. In that case, first establish how the coupling is supposed to work – for example by examining a lens known to have no problem – and then check the suspect lens for anomalous behaviour.)
For a more thorough test, or if you need to document this problem (which may be the case if you have a dispute with someone that sold you a lens with a sticky aperture), do the following:
Put the suspect lens on a digital camera and shoot reference target. A grey card or any evenly coloured surface (including white paper) in open shade is perfect. Set ISO 200 and put the camera in aperture priority mode (A or Tv). Use a tripod if you have one handy, but handheld usually works as well. First shoot the card or paper with the lens wide open (that is a small f-number, such as f/2 or f/3.5). This is your reference image.
Then shoot the same card or sheet of paper with the lens stopped down to its smallest aperture (that is a large f-number, such as f/16 or f/22).
Compare the histograms from the two exposures. If they are more or less identical, then the lens stops down correctly. If they are more than 0.3 EV apart, then the lens has a sticky aperture. The more apart they are, the worse the problem is.
The three histograms on the left shows the result of doing this test. Image #1 is the reference image, taken with the lens wide open. Image #2 is from a healthy lens, stopped down to f/16. Since we are in aperture priority mode, the camera's exposure computer compensates for the smaller aperture by selecting a slower shutter speed. As you can see, the histogram is similar to the histogram of the reference image.
I now change to a broken lens with sticky aperture blades. I then take another reference image with this lens wide open. I don't show this reference image here, as it is almost identical to image #1. The test image taken with the broken lens stopped down to f/16 is shown as image #3. The camera's exposure computer expects the lens to stop down to f/16, so it selects the same slow shutter speed as it did for image #2. However, the sticky blades prevents the lens from stopping down correctly, and we get an exposure that is about 2 EV brighter than the reference image. This tells us that this lens is broken and not stopping down as it is supposed to.
To fix a sticky aperture, if the lens has a mechanical aperture coupling lever or pin, first check that the coupling is not shifted out of position or bent. If the blades move, but slowly, you may to “exercise” the blades by pulling the lever back and forth. This may loosen the blades up and get things working again.
To fix a stick aperture on a lens with electrical contacts, you may try to clean the contacts on the rear on the lens and the camera body with a soft cloth and pure ethanol.
If this does not work, the lens need to be disassembled for repair. This is is work that should be carried out by a qualified technician.