[Leica] Test results of Hexar and M6 focusing

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

Conducting a series of tests on my Hexar RF, .72x M6 TTL, 35/1.4 asph, 50/2
and 75/1.4 Leica M lenses (all current production models)--using tripod,
tape measures, a ruler set at a 45 degree angle, and slide film--revealed
the following:

1. The min. focus distance possible for the Hexar's rangefinder is .673 m,
or 26.5 inches.

2. The min. focus distance possible for the M6 and both the 35/50 lenses is
.645 m, or 25.35 inches.

3. Because of this difference, the Hexar's rangefinder disengages for the
last 1.15 inches toward nearest focus for both the 35 and 50 lenses.  This
causes the image to be focused one inch closer than the rangefinder would
indicate.  Both lenses focus accurately at their nearest distance on the M6,
however, which is a full 2.2 inches (.7 m - .645 m) closer than their
specified min. focus distance capability.

4. The 75 focuses down to 28.5 inches on both camera bodies (within the
tolerance of both rangefinders), a full inch closer than the specified min.
focus capability of this lens (.75 m = 29.5 inches), but in this case
neither rangefinder achieves accurate focus: the sharp area of the resulting
image is actually 1.75 inches closer (at about a 45 degree angle, so
probably a little over an inch closer) than the area in which the
rangefinder patch coincided.

5. All three lenses on both bodies achieve accurate focus from .75 meters
through to infinity at all f-stops.

Conclusions:

My Hexar RF is perfectly accurate with Leica lenses through the published
minimum focus distance of .7 meters.  The problem is that the focus rings on
the 35 and 50 lenses actually turn beyond .7 m down to .645 m, and while the
M6 rangefinder can tolerate this extra two inches, the Hexar's quits after
an inch, and so images produced at _actual_ nearest distances on these
lenses with the Hexar are an inch closer than the rangefinder indicates.
The only way to correct the discrepency here is to make the focus ring on
the lenses only turn to .7 meters, or to allow the Hexar's rangefinder to
remain coupled through to .645 meters focus.

My 75 sample is accurate on both the Hexar and M6 down to the published
minimum distance of .75 meters, but the focus ring on the lens turns another
inch closer, and in this case the resulting focus is actually another inch
or two closer than the rangefinders indicate (and here neither rangefinder
decouples as it is within the tolerance of both).  The remedy here would
seem to lie with the lens, which needs to either be calibrated to achieve
accurate focus through the entire throw of the focus ring, or else forced to
stop turning closer than .75 meters.

Regards,
Dan