Cleaning the aperture diaphragm of a Nikon AF 50mm f1.8

In 2008, I bought a 50mm AF f1.8 Nikon prime to replace a 50mm f1.8 AI E-series manual focus lens that I had been using on a D40 body. This seemed to be a cheap way (about $75, second hand) of getting a relatively fast normal lens that would meter on a D40, which the old manual lens would not. I was very pleased with it until the grease from its focusing helicoid seeped into the aperture blades, making the lens unusable or at best unreliable except wide open.

When this happened, the symptom that I first saw was that the exposure settings provided by my camera's meter (and suggested by sunny-16) gave over-exposed pictures. The smaller the aperture, the more over-exposed the resulting image. Finally after about a month of trial-and-error, and wondering what was wrong, I noticed that when the stop-down lever (in the lens mount) was flicked with a finger, the aperture blades did not return to their closed position immediately, but instead seemed to stick. A sheen of liquid was also visible on several of the diaphragm blades. I gather from subsequently reading online comments that this is a relatively common problem. When I took the lens in to the store where I bought it second-hand (Service Photo Supply in Baltimore), they said they couldn't fix it except by returning it to Nikon, which would have cost far more than a new lens.

It turns out to be a relatively easy repair, requiring only a small phillips screwdriver and a supply of clean solvent (I tried ethanol and later white gas).

I adjusted the lens to infinity focus and f22 before removing the three screws from the metal lens mount (bayonet) and lifting the mount off. Under the lens mount are three loose parts: a thin metal washer, a cog for the camera to drive the autofocus mechanism in the lens, and the aperture ring. If the lens is turned over (pointed up), all of these will fall off (perhaps along with a bushing for the cog which is about the size of a large pin head), so it is important to note their positions before removing them inadvertently, as I did. (I had to replace them by trial and error after letting them fall off without noting their positions.) Next, I removed the lens assembly by unscrewing the four screws and removing the two L-shaped tabs that hold it into the focussing helicoid. Then I removed the rearmost two optical elements with a lock ring on the back and the spacer between them. Both go convex side towards the rear of the lens and fell out with a little teasing from a folded corner of paper. Next I wiped off the grease that I could see from the outside of the lens assembly. Note that there is a little hole in the back of the lens assembly into which you can insert a pin or unbent paper clip to flick the aperture blades open. If they are clean and dry, they should return to their closed position on their own (without the assistance of the return spring on the lens mount). Mine were stuck badly enough that they did not return on their own and had to be pushed both ways (open and closed). I spent a while trying to open this lens assembly up, which I assume requires unscrewing the front lens (objective) retaining ring with a rubber stopper or friction tool. I got a good purchase on it with a lacrosse ball wrapped in a latex rubber glove, but applied as much force as I was able to without budging the retaining ring. Since the lens was unusable anyway, I figured I had nothing to lose and immersed the entire lens assembly in a cup of clean ethanol. I used 190 proof (95% pure) lab grade ethanol made by Pharmco; I think it was their ACP/USP grade, whatever that means. I would be leery of trying denatured alcohol from the local pharmacy but grain neutral or methylated spirits might be all right, if you don't have access to a chemical stockroom. I dunked the lens assembly in a clean cup of alcohol, waited for it to fill, then took it out and flicked the stop-down hole with a paper clip a few times. I repeated this half a dozen times or so, with one change to a clean new cup of alcohol, and then left the lens to dry  (objective up) in the sun. The aperture blades unstuck almost immediately, but did not return to their closed position by themselves until they were dry. Actually the first time I immersed and dried the lens assembly I left it facing straight up in the sun and a very faint whitish residue remained in the center of the rear element when it was dry. (Not actually the rear element, because I had already removed that, but the third element forward.) So I tried the immersion procedure again taking care to leave it to dry on a slight angle (this time in a cardboard box warmed by an incandescent lightbulb) so that any remaining residue would collect at the edge of the lens. This worked nicely. It dried in about an hour, though I left it longer to make sure that there was no liquid caught anywhere invisible inside. I also flicked the aperture blades every so often with a paper clip and propped them open for about half an hour of the drying time. I reinstalled the assembly in its helicoid, screwed in its L-shaped guide tabs, then replaced the rear element of the lens assembly with its retaining ring. (On the first try, I got the lens element in the helicoid slightly wrong as the focus distance indicator was about a centimeter to the left, looking down on the camera, of where is was before. Trial and error should reveal the correct slot in the helicoid at which to start the lens assembly so that the focus ring indicates correctly.) Finally, I reinstalled the autofocus drive cog, aperture ring, and washer in that order, and replaced the lens mount with its three screws. Assembly is easiest with the aperture ring in the f/22 position. Nothing should require force, though it helped to depress the spring blade that provides a click-stop for the aperture ring in putting the aperture ring back on.

Reinstalled on the camera, the lens metered again and showed no signs of the repair.

The first wash only lasted a month or so before it started gumming up again, so I tried again using white-gas (naptha) instead of alcohol. I was worried about the purity of the gas (Coleman fuel), so I tried unsuccessfully to immerse only the diaphragm itself, not the optics. The drying gas did leave a slight iridescent stain on the rear-most lens (actually the 3rd from the rear, as I had removed 2). This cleaned up easily with an alcohol swab before reinstalling the rear two optical elements.

If I were to repeat the process, I would use first a white-gas bath, then an alcohol bath to clean up the residue.

So far (2014) there has been no recurrence of the problem; still using the lens....