[Leica] Konica facts

[Erwin Puts <imxputs@ision.nl>]

There has been much discussion about the compatibility of Leica lenses 
with the Hexar RF body. Most people, including all of the magazine 
writers have assumed that the study of the bayonet mount (that is does 
the Leica lens fit onto the Hexar bayonet) is sufficient to declare that 
lenses and bodies can be safely intermixed. Problems have been 
encountered and have been discussed as tolerance issues. As far as I 
know, no one has extended the analysis to the most crucial part: the 
back focus or the distance from bayonet flange to film plane. After 
measuring it  and checking with the Konica people a most surprising fact 
emerges.
The back focus of the Hexar RF is 28.00mm with a tolerance bandwidth of 
0.06mm!
The Leica data are: 27.80mm with a max tolerance of 0.02mm.
The first observation is this. Leica tolerances are 3 times as narrow as 
the Konica ones (0.02 versus 0.06). Wonder why the Leica is expensive? 
This small difference in tolerance is a hefty task in production 
engineering.
Most importantly however is the conclusion that Leica lenses cannot be 
used with any degree of confidence or performance on the Hexar. The back 
focus difference of 0.2mm and that is much too large for even  a modest 
demand on optical performance. It simply means that the leica lens 
focusses 0.2 mm short of the film plane. A distance that is wide enough 
to kill any idea that we are talking about precision optics or 
engineering.
Nor can we use Hexar lenses on leica bodies, now the Hexar lenses will 
focus behind the film plane by 0.2mm.

Why then are many users of Hexar bodies with Leica lenses happy?
Pick anyone of these explanations.
One  happens to have a Hexar where the max tolerance all are in one 
direction, which might bring the effective back focus to about 27.90 and 
when stopping down or using a wide angle the difference is covered by 
DoF.
The expectations about optical quality or the demands on the picture are 
quite low.
The topics photographed are quite tolerant for uncritical focussing.

I find it remarkable that this topic has not been discussed as it is the 
key to understanding the Hexar/leica compatibility issues.


Erwin