adjust focus of Canon fd 35-105 f/3.5 zoom lens

fstop · Joined: 11 Apr 2010 Posts: 29

Last night I gave it another try. Note that I am doing this sort of a thing for the first time on an fd lens am proceeding slowly. I have, however, opened and reassembled a USM zoom lens once successfully - needed to clean an inner element.

So, last night I pulled the focus rubber ring grip all the way back till it touched the zoom rubber ring grip. That revealed the collar under it, from the adjuster screws (x2) till the front end of the collar. No three screws separated by 120 degrees! The only screws I see are the two on the brass adjuster.

Also, I checked the focus carefully and I see what Unclemack meant by an inner adjustment being required if infinity focus is off in a different way at both ends of the zoom. Moving the adjuster brass clip is not going to sort the problem out, in fact, fixing it on end will make the error larger on the other end of the zoom.

So, next up is the procedure on how to do the inner element adjustment. Anybody know how to proceed?

I see two sets of screws around the lens barrel. One set is on the ring that has the mount lock button. The other set is between the aperture ring and the zoom ring, on the collar which has the orange zoom index mark. I am expecting one of these sets are the starting point.

balazer · Joined: 09 Sep 2012 Posts: 7

To make the Canon FD 35-105 mm f/3.5 lens be parfocal, more than likely you need to adjust the back focus (lens-to-flange distance), and nothing else.

Just in front of the aperture ring is a black plastic ring with "35-105mm" in green letters, an orange line, and a serial number in white. It has three Phillips head screws with the heads painted black. Remove the three screws. The black plastic ring can now slide forward, revealing four more Phillips head screws. With the lens pointed downwards, loosen those four screws. They'll be tight, so push in with a lot of force so that you don't strip the heads while removing them. With those four screws loosened, the lens's mount and aperture ring are free to slide up and down. If you didn't have the lens pointed downward, the mount could slide off completely, leaving you to figure out how to slide it back on. So don't remove it. You just want to be able to slide it up and down a bit, and then re-tighten the screws when you have it in the right position.

Mount the lens to your camera. Set the aperture to the widest setting and the zoom to the maximum. Focus on a distant object. Set the zoom to the minimum, and adjust the lens-to-flange distance until the picture has sharp focus. Note that with the four screws loosened, the mount can tilt, so be careful to keep the mount's axis aligned with the lens's axis while you're making your adjustments. If in doubt, check for equal focus at the four corners of the image. Repeat these steps until no further adjustment of the lens-to-flange distance is required to have sharp focus across the zoom range with the focus ring in the same position. Re-tighten the four screws that hold the mount in place. You'll want them to be quite tight, as they are supporting the lens's entire weight.

dualmp · Joined: 02 Jul 2011 Posts: 25

@fstop, did you managed to fix the focus? I am having the same issues as you and had tried balazer's suggestion but it didn't work.

pfogle · Joined: 03 Jul 2007 Posts: 11

Thanks guys - I got one of these with focus shift and bad decentering. Used balazer's tip to get it near to par-focal. It still has some residual field curvature, and a slight focus shift, but is hugely improved.

One thing - my lens has 6 screws holding the mount section, not 4. Whole job took about 1 hour, max.

I'm no expert, but following the instructions above, I got a great lens from one that I was ready to junk!

Herbert · Joined: 07 Sep 2023 Posts: 26

Hello to all!
I would like to change front glass of a Canon FD 35-105 /f3.5
from a scratched to a mint one (from a non working sample of course).
Will there be any alignment in the Y/Y and Z axis be needed?
Many thanks, all the best. Herbert