Oily aperture blades is one of the maladies that can effect old camera lenses. Oil gets onto the blades when the grease in the focus gearing breaks down and leaks. Age and heat are typically what causes the grease to break down i.e. a lens left in a car on a hot sunny day. The blades should be absolutely dry, they do not need lubrication.
The primary symptom of oily blades is usually over exposure on shots where aperture is set smaller than wide open. So for example with a 85mm f1.4 lens, wide open is f1.4. If you set f5.6 on the camera and take a picture it comes out overexposed for no good reason.
What is happening is that while focusing, the camera holds the lens wide open so you have a bright viewfinder and the AF unit can do its job well. When you press the shutter release the camera closes the lens to the specified aperture and takes the picture. With oil on the blades, the blades don't move or not quickly enough to get to the correct position. The result is that you end up with an aperture more open that what was specified on the camera.
There are a few ways to check and see if this has happened to a lens.
- Visual inspection is the best way. Obviously if you can see oil, you have identified its existence. Remove the lens from the camera and shine a flash light into the lens and look down on the blades. In most cases you can get the best view looking through the front element but you can also try the rear. Oil will appear as a circle or little triangular wings. Clean blades will look consistent and dry. Patterned discoloration is generally a sure sign of oily aperture blades. In large aperture prime lenses this technique is usually very easy to do. For zoom lenses it can be very more problematic.
- Check "snappiness" of the blades. With the lens off of the camera, pull the aperture actuator back to open the blades. It should retract smoothly without stickiness or inconsistent resistance. When released, the blades should snap closed. If they stick in place or close sluggishly, this may indicate oil on them or a broken return spring.
Some lenses will operate fairly well even with oil on the blades and in other cases the blades might become completely stuck.
The difficulty in cleaning blades varies considerably between lenses. In any case for this job you will need the right tools, instructions and care. If you decide to give it a try, take good pictures or make a video of the disassembly, so you know how to put it back together. Work over a tray or have some way to capture parts if they drop from your work or a spring fires them off.
Of course, for rare or valuable lenses please consider a reputable repair shop. I have heard good reports about Keh’s repair service