Canon LTM Canon 28mm 2.8 LTM focusing issue

[Marsupilami]

Hi!

I have an issue with a recently purchased Canon 28mm 2.8 LTM lens.

Trying the lens (but not yet tested with a film) on both my M2 (CLA’ed) and my IIIf with comparisons with my other lenses (Summaron 35mm 2.8 and Summitar 50 2) show discrepancies on the distance indication ring.

I mean, focusing on an object 5 meters away gives an indicated distance of 3 meters
Infinite is wrong, I focus on infinite around 6 meters, and if I continue to turn to the infinite lock, then focus is wrong.

My Summaron & Summitar behave as expected.

The seller said he used the lens for 3 years and never had a problem and advice first to test with a film.
I can do a roll but I’m afraid I will just loose valuable film…

Lens is clean but has been opened.

Questions:
- do you know or have you experienced this kind of discrepancies on this Canon 38mm (aka is it a known problem)?
- if not, what would be wrong or misaligned?

Thanks!

 

[dexdog]

This sounds like a job for Sonnar Brian! (Add dramatic fanfare with trumpets here). Lens is apparently back-focusing signifcantly. Probably need to increase the shim to correct. Sonnar B is the best source as to how to do so.

 

[Sonnar Brian]

Are you using the same LTM to M adapter with this Canon as you are with the Summaron (if it is an LTM)?

The thickness of the adapter must be precise for a wide-angle lens. The cheap Chinese adapters are fine for 50mm, where the movement of the optics is 1:1 with the Cam. With a wide-angle lens, the movement of the Cam is greater than that of the optics. The thickness of the adapter is critical.

 

[Marsupilami]

I have the problem on my III f, screw mount, so without adapter.

Same problem if I use the original LTM/M adapter of my Summaron

 

[Sonnar Brian]

It may have been misassembled- someone took it apart, and started the helical at the wrong position. That would explain the Distance scale being off. If the distance scale was correct, and the actual focus was off- that is corrected by adjusting the Shim that controls the stand-off the the optics from the film plane.

 

[Deleted member 65559]

I recently bought a clean example of the same lens. I use a cheap Fotodiox 28/90 LTM to M adapter I had from back when i owned a VC Color-Skopar 28mm f3.5. My example focuses well.... no issues.

 

[Marsupilami]

Update: I have managed to fix the problem.

Indeed the issue was coming from a wrong reassembly of the rear helicoid (the one driving the telemeter). After a bit of try and test, I managed to find the right spot and now infinity is on focus on infinity-lock and distances are correct on the focus ring.

For the record, I have also found some informations on old Canon lenses where, like Sonnar Brian mentionned, infinity can be finely tuned using an inner shim. Check here (in Japanese) mglss studio photography

While opened, I also took the opportunity to make a good cleaning and lubing, so it's almost as good as new

 

[aw614]

I had a similar issue with my Canon 35mm 2.8 LTM lens that I bought on ebay in "as is" condition for a really good price, initially found two areas where the helicoid would start to thread, and one appeared to work, I just coudnt focus to infinity. Seller sold it as having a bent helicoid and I thought it was too...

But unknown to me, was one more area that also could thread the helicoid and now it works great

 

[MarkWalberg]

.....For the record, I have also found some informations on old Canon lenses where, like Sonnar Brian mentionned, infinity can be finely tuned using an inner shim. Check here (in Japanese) mglss studio photography...

Thank you very much for the link to this site. There is some very useful info there for CLA of some Canons and others. It is very well done indeed. (Google will translate the Japanese to another language.)

 

[Deleted member 65559]

A gem of a lens...especially @f8 1/125th & hyperfocal distance
makes it a great point & shoot
I use it on both my CL(film) & M4