Re: [Leica] Re: Hexar Back focus

[John Collier <jbcollier@powersurfr.com>]

I can only speculate but perhaps the flange to film distances are the same.
The difference may be in the measuring technique. If Hexar measures from the
bayonet flange to the pressure plate, this would give a greater distance
than Leica's  nominal 27.8mm which is to the front surface of the film.

John Collier

> From: "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
> 
>