M6 focus good with some lenses, not with others

I've had for 10 years a nice second hand M6 (ca. 1992) in which I have had

complete confidence. In the past, it has accurately focused two examples of

Noctilux at full aperture (two because I was trying to understand the focus

shift, but it turned out to be the nature of the Noctilux, not the camera's

ability to focus). I considered this a very good test of its focus adjustment,

and subsequently any slightly out of focus errors I attributed to my haste or

lack of care.

 

After my recent experience with a ZM Sonnar which I did not keep, I tested my

other lenses, using a printed target, tripod, 25asa film etc. Two 50mm lenses

(Summicron and SummiluxASPH) and a 90mm Elmarit-M focus very close to spot-on at

2.5metre. But two 35mm (Summicron pre-aspherical and SummiluxASPH) both front

focus by something like 250mm or a bit more.

 

I doubt that both 35mm lenses are faulty. I bought both new. Can a body be out

of adjustment in such a way that wide angle lenses focus poorly and longer

lenses focus well?

Unlikely that the body is at fault. The RF only sees the slope of the focusing cam on the lens and every lens is different. The RF in the body is consistent and doesn't change with evach lens. The RF rider / arm follows the slope of the focusing cam and the cam is suposed to be ground and set for each lens. The lens itself is frequently a slightly different focal length that what is on the fron of the lens. My 90 Elmarit v1 is somewhere around 88.7mm if I remember what is inscribed on the lens head inside the mount. The mount is individually adjusted for that particular lens group and won't work properly with any other.

Well, shorter focal length lenses are more sensitive in errors in lens flange to film plane distance than longer lenses. (While the DOF is larger on the subject side, it is shallower on the film side.) So it could be that said distance is off, and someone calibrated the RF to correct for it, at least with longer lenses. There are about different focusing calibrations in a Leica camera and lens pair, all have to be right.

Somehow I lost a 7 in there. There's about 7 things to calibrate to get accurate focus.

It was only when I got my M8 that I realized my 50 Summicron focused well beyond the proper distance (about 10 cm at 1.5 meters) thus preventing proper focus from F4 up! My Noctilux has a enormous focus shift (only in the center of the image) when stopping down until 5.6. These were almost unnoticeable when using fast film, which is thick enough to compensate for the focussing discrepancies, but not with a CCD! Both my 35s (Cron and Lux aspherical) are perfect, as well as my 90 Cron (which is difficult to focus wide open anyway). It is essential to have all the optics tested if you intend to use them at their maximum performance.