[Leica] infinity focus

["Michael Darnton" <mdarnton@hotmail.com>]

  Might I suggest a test for those who are concerned about the focus of 
their lenses and rangefinders? I put a yardstick on my desk, with a pencil 
pointing to any specific point near the middle, then shoot a test shot of 
the yardstick, from one end, from any distance, at a relatively wide 
opening, while focusing on the point where the pencil is pointed. This is 
pretty easy, because the two images in the rangefinder form an X, which you 
can move up and down the yardstick by focusing the lens, until the center is 
right at the pencil-indicated spot.
  When you develop the film you can then use a magnifier to assure yourself 
that the lens has focused where you intended--the numbers on the yardstick 
drift in and out of focus through the focused-on area in a way that's very 
easy to see exactly what's going on. This is a much more telling test than 
just popping off a shot at infinity, because you can actually *see* the zone 
and placement of focus.
  Of course, for this to work, YOU have to have good enough eyes to focus 
the camera correctly :-) When testing this, I try to use a distance that's 
meaningful for the kind of photos I do (which are almost never at infinity, 
anyway).
  The results and solution can be discouraging. First, there's the ideal 
distance from the lens flange to the film, hopefully set at the factory (WAY 
off on my IIIA, which I intend to fix when I get around to it); if you're 
using LTM lenses on M cameras there's the thickness of the adapter (more 
below); there's the calibration of the lens; the calibration of the camera 
rangefinder. There's also the mis-match of the mount to the optics (remember 
those tiny little numbers near the infinity mark on some Leitz lenses?). I 
have one lens (non-Leitz) which is hopeless in this respect--fine at 
infinity, terrible at three feet, progressively worse through the range 
between.
  I have four LTM-M adapters. Three are new, and measure .96mm thick, one is 
Leitz, measuring 1.04mm. I don't know if 1.00mm is ideal, but at least 
there's a .08mm variation in this group. That means a lot to a wide-angle 
lens (somehow that idea of greater depth-of-field makes sense to me only 
until I start looking closely--with a W/A it's not really greater--it's just 
harder to see the problems because they're smaller--my 15mm Heliar really 
doesn't have that much DOF, when it comes right down to it!). After a little 
playing around, I think I've got everyone paired up with his right partner. 
I have one lens which is still a problem, and a fix was attempted by a name 
you all know. Suffice it to say the fix was crude, poorly done, and I'll 
probably re-do it sometime, better, leading me to comment that I think 
there's work out there for people who want to learn to fix things right (But 
that's a topic for another thread. . .)
  --Michael Darnton
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