Canon EF lens repair: 35-70mm f/3.5-4.5 won't focus at infinity

I picked up this lens for a song and now I know why: it won't focus to infinity. At most I can focus on objects about 3 feet away, but the distance scale is completely off and says that it's focused at about 8-9 feet. Objects at infinity are totally blurry.

 

Does anyone know how to adjust focus on this lens? It's one of the earliest EF lenses and makes for a great "adjustable prime"/"adjustable normal" lens for bright light, as Mike Johnston calls lenses with narrow zoom ranges.

 

I was reading this blog post by Roger Cicala today that says that Canon EF lenses are adjusted for infinity focus solely by the thickness of the mount, unlike Nikon lenses which use shims. He writes, "Canon bayonet mounts come in various thicknesses, so rather than shimming they simply choose the correct thickness mount when they assemble the lens." Does that mean I have to buy a replacement mount that is thinner than the one I have? I checked the auction site and they don't sell mounts by different thicknesses, so I'm not sure what's going on.

Can you manually focus it to infinity? And you are sure your diopter has not been bumped out of focus? This isn't an expensive lens, I see KEH is selling used versions of this lens (graded in excellent condition) for $59-$62 and free shipping, so it is not worth sending out for repair in my opinion.

 

I found this, not in English though.

I would assume the lens was originally manufactured with the proper lens mount and the problem now is with the AF mechanism. As stated above, repair is not going to be cost effective, so you can try to repair it yourself using youtube instructions, use manual focus, or get another lens from KEH that works.

Something has slipped badly in the AF mechanism. Nothing to do with the mount.

 

The early 28-70 zoom is a much better regarded lens than the 35-70.

Can you manually focus it to infinity? And you are sure your diopter has not been bumped out of focus? This isn't an expensive lens, I see KEH is selling used versions of this lens (graded in excellent condition) for $59-$62 and free shipping, so it is not worth sending out for repair in my opinion.

 

I found this, not in English though.

 

No, I can't focus manually to infinity. I was checking the focus on a digital body by checking live view, and the farthest it focuses is 3ft. I'm going to try fixing it myself because it's so cheap, so thanks for that link!

I would assume the lens was originally manufactured with the proper lens mount and the problem now is with the AF mechanism. As stated above, repair is not going to be cost effective, so you can try to repair it yourself using youtube instructions, use manual focus, or get another lens from KEH that works.

 

Thanks for the diagnosis. It's good to know I don't have to buy any spare parts.

If I had to guess, I'd guess that some debris that has a) either worked it's way in over the years, or b) broken off inside, and has wedged it's way into the focal track. I've certainly had that happen to other lenses in the past. Regardless of the source though, at least the disassembly and diagnosis will be fun! ... just make sure to get yourself some JIS screwdrivers (they are NOT phillips) before you dig in, else you'll likely have stripped screws to deal with too.