Nikon 28mm f2.8 (AI-s)

Nikon 28mm f2.8 AI-s

Problem: stuck aperture and some fungus

The 28mm f2.8 Nikon lens is a solid build wide manual focus lens produced after 1981. This version is the last manual focus 28mm lens by Nikon. The lens uses a quite special optical design. More info you can find here: kenrockwell. This particular lens serial number puts it's date of birth somewhere at the late 80's or early 90's.

The lens had a stuck aperture and some fungus specs on on of the elements.

Fig 1 - Nikon 28mm 1:2.8 Ai-s with stuck aperture

So first we need to get to the aperture and. Most of the time the easiest way is by removing the mount and accessing the lens from the back.

Fig 2 - Nikon only used 3 screws on this version, well done Nikon. Note that only the mount screws need to be removed. Don't remove the black cover screws.

Fig 3 - The mount ring viewed from inside. This is a good time to wipe the "Auto" lever mechanism and remove any old dirt and grime.

Next you can remove the aperture ring assembly (the silver set ring, the stopper black ring and the aperture ring it self). Note the orientation of the silver set ring, as it can be placed back the other way around. Now you can also unscrew the back element groups. They all come out as a single piece which is great.

Fig 4a - Note the silver aperture setting  ring. The tab (on the right) should engage the slot in the plastic ring. The two screw heads connecting the tab to the ring should point inward.

Fig 4b - The "stopper" black ring with the stopper tab

Fig 4c - The aperture rings removed and the back lens groups removed. Now there is access to the aperture blades.

At this point you can close the aperture by moving the upright pin. Ones the aperture is closed oil can be removed by wiping it out with cotton swabs. I use White Gas as a solvent. Lighter fluid can also be used. 

At this point maybe you're lens is back working with a snappy aperture. If so, you can start to assemble it (In this case I believe in "don't fix what is not broken"). But in my case I needed to remove also the front lens assembly to clean the fungus, and clean the aperture blades from the front as well.

Move the focus to is minimum focus distance do the front barrel will extend and the tine set-screw can be visible. Remove the screw and now you can unscrew the whole front end (the filter ring).

  Fig 5 - With the barrel extended the holding set screw can be removed.

Fig 6 - With the front end removed, the front element group can be unscrewed.

Now we can simply unscrew by hand the front element group and clean it from inside (as I did to remove the fungus) and clean the aperture blades some more.

Fig 7 - The front element group - Cleaned. 

Fig 8 - Removing the old oil from the front of the aperture blades.

Thats it, at this point you can assemble back the lens tracking back the steps. Make sure you dust off the lens elements before screwing them back in. Remember, if you didn't tough the glass, most of the time you don't need to wipe it, just use some compressed air or a rubber blower to blow of dust.