[Leica] Leica back focus new info

[Cummer Family <cummer@asiaonline.net>]

Dear Luggers,
On June 7 I asked:
>Anybody know what is the bandwidth for Leica backfocus? Is it 27.8 mm +/-
>0.01 or is it 27.8mm +/-0.02 - the former yielding a bandwidth of 0.02 and
>the latter a bandwidth of 0.04??
Not receiving any answer from any Luggers I telephoned the Leica Repair
department at Schmidt this morning and spoke to Mr. Yu who is the senior
repair technician. I asked Mr. Yu to confirm the back focus on the M6 from
bayonet face to film plate was 27.8mm as reported on the LUG. Mr. Yu dug
into his technical sheets and told me that the back focus was 27.95mm +/-
0.02mm. I said that could not be because I was sure that it was 27.8mm. Mr.
Yu said no - the back focus was 27.95 without film and estimated to be 27.8
with film. This information, assuming its accuracy, shines some light on
the inconsistency between being able to use a Hexar RF (backfocus 28.0
+/-0.03mm) with Leica lenses when the lenses shouldn't be able to focus
accurately with a back focus difference of 0.2mm. Now, if the specs which I
was given are correct a perfectly in spec M6 will have a back focus of
27.95mm and a perfectly in spec Konica will have a back focus of 28.00mm -
a difference of only 0.05mm. 
I speculate that the few people who have had difficulty focussing Leica
lens on Hexar bodies either have Leica lens slightly out of spec (like my
new 75 Summilux now on its way to Solms) or a Hexar RF body at the extreme
of its tolerance of 0.03mm yielding a maximum difference of 0.08mm from the
Leica spec. I have now decided to stop wasting film testing my lenses on my
Hexar and go out and take pictures, (amber rain warning in Hong Kong
tonight great wet street reflections). I am relieved that I have satisfied
myself (if no one else) about the inconsistency between real world use and
what the original numbers on the LUG predicted.
Cheeers
Howard.