Lately, I've been getting poorly focused pictures with my 75mm lens (a Cosina LTM, with an M adapter). The zone of sharp focus is often 6 inches behind where I intended it to be (i.e. the back of a sofa, rather than the eyes of the subject, who is resting head on sofa), even a f4, f5.6. I am able to focus my 90mm lens quite precisely, as also the 50mm.

Can a lens go out of focus alignment? What's going on? Ideas and guesses welcome, as is information.

-- Mani Sitaraman (bindumani@pacific.net.sg), October 28, 2001

Answers

Yes but it is more common for the camera's rangefinder to need adjustment. It is simple to test for. If all your other lenses focus fine except this one, it is the lens. If the lens works fine on other bodies but not this one, then it is the body. I have had one 50/2 that needed its focus cam adjusted. It was fine when I only had the one lens as I adjusted the body to suit. As I added lenses though, it soon became obvious somthing was amiss. It is not very expensive to adjust on Leica lenses but I cannot comment on Voigtina lenses.

Cheers,

-- John Collier (jbcollier@powersurfr.com), October 29, 2001.

More likely is was manufactured / adjusted from factory outside acceptable tolerances. Have you carefully checked the infinity setting on the lens to see if a very distant object lines up at with the double image?

-- Andrew Schank (aschank@flash.net), October 29, 2001.

That your other lenses work fine tells you it's not the rangefinder. That the lens can form a sharp image tells you its back focus is probably ok too (you could double check that by putting a groundglass in the film plane.) That leaves the focusing cam and the adaptor. If the lens could focus properly at one time but doesn't now, try removing and re-seating the adaptor. Then run the following check: set the lens to its minimum focus (one meter?) put a piece of tape on a mirror, and focus on it by moving the camera back and forth. Then step to the side and refocus on the image of the camera. The scale on the lens should now say twice minimum focus (2 meters?). finally, focus on a cloud and make sure the scale in the lens says infinity. If the lens passes this check , you are probably back in business, unless the cam has an irregularity somewhere along its length. If the lens flunks this check, it needs service - probably regrinding the cam- or if still in warranty,

-- david kelly (dmkedit@aol.com), October 29, 2001.

that was supposed to read "if still in warranty, replacement".......

-- david kelly (dmkedit@aol.com), October 29, 2001.

Mani: in the check I suggested, forget the step to the side, which could introduce error. Just ignore the tape and refocus on the camera's own image..........

-- david kelly (dmkedit@aol.com), October 29, 2001.

Was this 75mm lens ever right? I had an old Canon 135 lens that had the RF coupling cam ground inaccurately; it was off, but it was always off. If the lens has changed, something must have come loose inside (optics or RF cam).

rick :)=

-- Rick Oleson (rick_oleson@yahoo.com), October 29, 2001.