fixing a stuck aperture - win or bin

I bought a Canon FD100mm f2.8 a while back and it worked fine for a time then I observed it would not stop down reliably. It was clear there was oil on the aperture blades and so I reluctantly decided that the only option was to either bin it or fix it ... once I'd decided that it could only be win or bin I decided to give it a go.

WARNING: this is fiddly job and requires some competence with tools

This post serves to add a few things to this great post I found on photo.net thanks to gnashings. He also references this post which while about the FD50f1.4 is also helpful as the designs are very similar.

There are variants, but on my model the front "dress trim" is not unscrewed, its levered up ... which was perplexing at first and took some bravery. Essentially that last step before the lens is actually metal and holds the front element into a "group". The next step after that is the beginnings of the pastic. You can hear / feel it is different when you touch it with a small metal screw driver.

so I inserted a small thin screw driver into that gap where the red arrow points, and with a clean icecream stick to provide a fulcrum point (allowing me to leaver against the lens and protect it as wood is softer than glass) I could lever that up and get in another slightly larger screw driver in and work around that levering it off.

Unlike that above post by gnashings I had no hole through which to poke a screw driver ...

This revealed the three screws that hold on that metal front (to the plastic).

you can see traces of the glue in there (and my ice cream stick).

With the fascia removed you can now take out the front element which just sits in the plastic group mount. You can also see one of the screws that you'll need to remove to get the focus ring off, and these also allow you to later rotate that grip to tune infinity focus .

I placed a clean tissue over the front then up-ended it (lens element facing down) so that it could fall out into my hand but not get grease all over it

you can now see the iris diaphragm (which in this shot is now cleaned).

Notice that the lens is on an adapter, this proved handy (I'd say crucial but because I haven't tried it without I can't be sure) to move the levers and have the lens mount "rotated". Recall that the lens rotates within the mount when mounting an FD (check it if you're unclear).

I decided to go in through the front which is a very tedious process ... I don't recommend it as matching up

• the moving group inside the helicoid
• the two iris coupling levers
• and getting focus correct

took me half a day. However I did get it and all it took was perseverence (although that bin kept looking tempting)

Notes:

I should have photographed it, but as my hands were covered in grease I got carried away working and didn't, however the focusing ring (once those screws are removed) is held in by a lug on the side which while allowing it to move, prevents it from coming off. One has to keep rotating it till you find a keyway cut into the chassis of the lens. When you rotate it to that point it will slide up and away from that. As when those screws are removed it rotates freely and does not connect with the lens focus helicoid.

Take careful (and I mean that) note for where the outer helicoid is in relation to the body before you unscrew it as one needs to to when accessing from the front. By this I mean perhaps marking it with a score and observing the number of threads visible in the recess in the outer body (this will be clear when you see it). You will need to return it to more or less this exact position and you'll need (no matter what) to be tuning focus later if you want that focusing ring to ever line up with what the camera sees.

I had trouble screwing the lens back together, as if something was fouling. I discovered that actuating the open and close of the adapter seemed to release that friction and it then moved down with turning the focus part of the helicoid.

Have some small powerful magnets handy for screws.

Not having gone in through the back I can only say read his (the one linked above) post (and indeed the other comments) carefully.

Conclusion

My lens is now functioning perfectly so its win win not the bin.