2026-04-29 Image 28 mm f/2.8 Repair

This Auto Image 28 mm f/2.8 lens in Canon FD mount seems to have a aperture shutdown problem. I have other lenses just like it, but feeling brave one day I decided to take it apart to see what was jamming it up.

It was largely a disaster. I had set up to take images of each step. After taking one image of just the mount I decided to take out the three screws that hold the Canon FD mount onto the back of the lens (black screws at a clock’s 1, 5 and 9 roughly). My first screwdriver turned out to be so worn it wouldn’t grip the cross headed screws, After a trip to the basement for my complete set of small screwdriver bits (I needed a Japanese 000) I found the screws really tight. So tight the tip of the screwdriver sheared off. After cleaning out the remnants of the tip with a snap-off knife tip I finally managed to remove all three black-headed screws.
Clear sailing you think, let’s see the steps you say, well I was too busy fighting with the mount to take time to document each step.
Things you have to be careful about include—

A very small spring loaded pin that fits loose in the lens (at 3 on the clock face) has to be watched carefully
there are two linkages from mount to lens: one has a sharp hook (connected to the black projection close to mount screw at 5) and one is straight (connected to smaller black tab in curved slot close to black mount screw at 9)
where the f/stop is set may enter into lining the linkages up again
where the mount’s breech ring is positioned may make a difference…or maybe not

As an indication of how much fun this is, I reassembled and then took the lens apart at least three times. Every time it worked when I checked—before replacing the screws—and twice it stopped working after tightening the screws in place.
The lens aperture setting is done by the straight linkage. The hooked linkage does the shut-down to the aperture the straight linkage is set to. The aperture is spring loaded to open after the exposure. The hooked linkage is spring loaded to reset after the camera slams it.
So the action is—

set aperture choice on lens
fire camera, which shuts the aperture down
aperture pops open again after shutter closes

The problem with this lens is the spring that resets the aperture to wide open—wound around the optics in the midpoint of the interior—is weak. If it was just a bit stronger the lens would be fine. Similar action happens with aperture blades fouled by oil or dirt. The increase in friction slows the action and sometimes even sticks so the aperture does not fully open and close during the exposure.
Strangely one of the things that helps can be working the action. You don’t have to mount the lens and use a camera to work the stop-down linkage.

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