RE: [Leica] Re: Hexar Back focus

["Dan Honemann" <ddh@home.com>]

> I asked Mr. Leung to look at the Hexar service manual and read with my own
> eyes that the back focus is 28mm +/- 0.03mm - not the 0.06mm which Erwin
> reported on May 9 2001 to the LUG:

Here's what I still don't get.  Erwin says the backfocus measure for the M6
is 27.80mm with a max tolerance of 0.02mm.  Even if the Hexar RF is 28mm +/-
0.03mm, and slanting both  tolerances toward the middle, we'd have the M6 at
27.82, and the Hexar at 27.97--a difference of .15 mm.

What I don't understand is precisely what effect one would expect to find in
images produced by two different cameras with a .15 mm difference in
backfocus when shooting through the same lens?

Henning Wulf has stated that a backfocus difference of .2 mm would cause a
35 mm at f1.4 to actually focus at 6.2 m when the lens should be focused at
infinity.  His tests (and mine, with the 35/1.4 asph on the Hexar RF) prove
that this is not the case with our samples.  But perhaps there's a big
difference between a .2 and .15 mm discrepency?  And maybe my eyes aren't
able to notice the difference between the 35 focused, say, at 20 meters as
opposed to several kilometers.

Erwin says that this backfocus difference results in image degradation
comparable to shooting handheld at 1/30 sec. vs. using a tripod for the same
shot.  Maybe it simply boils down to this in my case: I honestly don't see
much (if any) difference in slides (projected; I don't display them for
folks under a loupe) I've shot handheld vs. tripod-bound using my
rangefinders, and so perhaps the same degradation exists in my Hexar images
due to the backfocus and it just doesn't affect me.  Who knows?

One might argue then, why shell out all that money for Leica glass?  The
answer isn't resolution, but all those other qualities--contrast, color
rendition, flare control, bokeh--that don't seem to be affected by backfocus
discrepencies or shooting handheld.

Dan