[Leica] LUG: TA's 21Magoo conversion

["Chris Quinn" <cquinn@mail.sjcsf.edu>]

Tom & Reinhold's 21 goggle conversion uses the goggles off of a 35; the
goggles are not computed for the 21, but for the 35. They produce a proper
'framing' in the viewfinder, but the 21mm lens focusses correctly only at
infinity. For practical purposes, the lens focusses accurately down to
fifteen feet, because of the depth of field of this focal length. Below
fifteen feet, mis-focus appears. In my own humble & unscientific test of a
21f2.8 pre-asph goggled 21, I found that the converted lens focusses about
six to twelve inches behind the rangefinder's 'correctly focussed' setting
at distances under fifteen feet, with the misfocus growing greater
(apparently) as the near focus point is reached.

Tom has a friend in Sweden who has computed a re-ground focussing cam, but
Reinhold has not yet tried a full-conversion with re-ground cam.

With the lens stopped down, of course, the focussing issue is reduced by
depth of field. The alternative is to 'compensate focus,' which simply means
always focussing the lens properly, and then underfocussing it. Not a pretty
solution, but....

Essentially, this means the lens is great for stopped-down work, or for work
at greater distances. For low-light and/or closer work, the converted lens
is a compromise. Until a re-ground focussing cam is available (Reinhold, are
you listening?), you have this choice: an unconverted lens which allows you
to focus through the finder, but not compose, or a converted lens which
allows you to compose, but not focus with complete accuracy.

Tom's original reasoning behind this conversion was the recurrent loss of
finders (have I got that right, Tom?); with the new Cosina finders, you can
buy three of them for the price of the conversion, whereas with the Leica
finders, you were limited to two.

Any other responses or clarifications to my post are welcome. As I
mentioned, the results of my test are not strict, and my estimates of
accurate focus may not be, well, accurate. But it should give some idea of
my own experience with a converted lens.

CQ

PS: Tom, if you would put some leverage on your Swedish friend, and then on
Reinhold, for that corrected focussing cam, I'd appreciate it.