Archived posting to the Leica Users Group, 2001/01/16
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]That's interesting and brings up a very ugly topic: film-plane focus. We all get worked up about "my Summilux" being off and this generation of this lens performs better than that. While that by some measure reflects truth, at the level of film-plane focus, it is not. We forget that whatever the optical merits of the lens or the rangefinder, the whole look can be changed by moving the plane of focus a fraction of a millimeter. How do we move the plane of focus? - -- First, we can boff the focus with the rangefinder. This is very easy, and can be done by turning toward the near instead of turning toward far. All helicoids and focus cams have play. If they didn't, your lens would be a $2000 fixed-focus monster. Think of the 200 parts in an M rangefinder. Add up all of the individual tolerances. I have some Rolaids handy. - -- Second, the lens can be calibrated to have an ultra-sharp center or very high sharpness all-around. The plane of focus is most often a parabola that can be made to meet the film at the center (for supersharp there and ok elsewhere), sharp in the corners only, or mediocre everywhere. This is on Robert Monaghan's web site somewhere. I only wonder what happens when you have a lens "collimated." - -- Third, the body focus of the camera can be off. This can be done via accidents that destroy the lens and leave the body "visibly" normal. That's why second- and third-hand bodies, of whatever make, have to be tested. Rigorously. Die-cast aluminum is bendable and so is zinc to some extent. Brass is very soft. It doesn't take much force if applied properly (like having a camera come straight-down on the lens) to throw this off. If you think otherwise, mount a very heavy lens on a rangefinder. Line up infinity. Flip it upside-down. Notice anything? - -- Fourth, your LTM adapter can be out of tolerance, or it can be worn. Self-explanatory. - -- Fifth, your film may not be completely flat. More than 2-1/2fps can cause sharpness problems, or so says Zeiss. Also depends on the size and configuration of the pressure plate. This, I believe, was the justification for keeping the bottom-load and back-door config of the M series. - -- Finally, any combination of the above. This is the "system tolerance." Since the early 1970s, the Japanese makers have been grinding the film rails to control the final tolerance. This means that the overall tolerance is under very tight control, but individual parts could be off by a lot. Before that, body focus was adjusted to the castings by hand. Leica professes (or at least did) to have every part within a tolerance for ultimate interchamgeability. I would be wary of either approach. One can lead to difficulty in replacing parts due to a larger tolerance in a particular part; the other can have lots of little tolerances adding up. Maybe I have just been lucky, and everything adds up right. My Hexar RF consistently produces better pictures with LTM lenses than my M3 does; and on bayonet lenses, it's hard to tell the difference. I would give the telephoto edge to the M3, because it's easier on my eyes, but I'm not seeing any noticeable difference in details at 2700dpi scans. But then again, I stop looking when I find a camera with good "karma" and everything adds up nicely. I think tolerances of whatever make have gotten tighter with better instrumentation and better quality control methods. The question I have is whether the new Ms are higher-precision than the old. In other words, with the same 90mm f/2 lens, would an M3 be better than an M6 0.85? I would guess not, unless you were shooting into the light. DAS "Rodgers, David" wrote: > There's an interesting comparison of the M6 and Konica rangefinder in the > latest Leica magazine. In one example both the M6 and Hexar bodies were used > with the same lens. The M6 photo appeared to be more sharp. That was stated > in the article. I make no assumptions regarding the objectivity of this > comparision. I just find it a bit interesting. Food for though, at least. > > It reminded me of a discussion years ago on the old Compuserve Photoforum. > The gist of it was that somebody packed some M bodies very tightly in a > suitcase. After much handling photographs taken with those bodies were not > as sharp. I had something to do with the alignment of the film plane. I > scoffed thinking it was nonsense. M bodies are built pretty well, after all. > But more and more examples were presented by people whom I trust know what > they're talking about. I don't discount such things as quickly as I used to. > OTOH, it's really difficult sometimes to separate fact from fiction. But > hey, isn't that what the LUG is for :-) > > Dave