[Leica] Rangefinder topics

[imxputs <imxputs@knoware.nl>]

Rangefinder topics.

I have hesitated for a long time to discuss the topic of rangefinder flare and 
accuracy. It is a theme that deserves some close reading and open mind 
approach. Let me start by noting that the rangefinder mechanism is extremely 
complicated as an optical and engineering construct. And without a firm grasp 
on its inner working, it is easy  to draw inferences that seem plausible, but 
are not.
The M3 finder is different from all other M-finders including the M2. The 
difference is not so much the outward specs (magnification and number of 
framelines), but the internal design. The M3 has been designed as a 50mm 
finder at maximum and the optical construction is such that the ray paths of 
the finder entrance window, the rangefinder patch and the illumination window 
for the frame lines all follow a separate path. This design is optimised and 
fully dedicated for its limited task: have a 50mm view, with a permanent frame 
line for the 50 and the additional, but separate frames for 90 and 135.
With the M2, a completely different design was necessary. The accomodation of 
the 35mm frame could not be squeezed into the optical and engineering 
parameters of the M3 construct and so the designers had to produce a different 
solution. Internally the M3 and M2 are totally different! To get the larger 
angle of view a smaller magnification was needed, and the impact this 
requirement had on the design it can hardly be understood. The permanent 50 
frame disappeared, and the finder, spot and illumination rays through the 
system followed a more complicated path. This path is partly the cause of the 
flare problem. Inherently the M2/4/5 finders are of the same design and share 
the same properties as the M4P and M6 finders. Still is is evident that the 
M4P and later finders have a higher sensitivity to flare in the rangefinder 
spot. The addition of the 28 frameline and the pairwise display of two frames 
at once necessitated another change of the mechanism. The mechanism of 
framemasking had to be changed and compared to previous finders the clarity 
and transparacy of the finder was enhanced. Any M4P and M6 has brighter 
framelines and higher transmission than the previous models. Both requirements 
could be met, but the internal meachnism had to be changed. During this 
process, the enigmatic condensor was dropped, not because of cost cutting to 
save the company as one observer has proposed, but becuase it had no longer 
any functional value. 
The tendency to flare under some conditions is a characteristic of the M4P and 
all M6 finders, but it is not absent from the M2/4/5 finders. It is however 
less pronounced and it is indeed almost absent in the M3 finder which is based 
on a different optical/mechanical construct.
Some observers persist in stating that this state of affairs is due to 
costcutting and cheapening of the finder. It is not. The current M6 finder is 
as complicated and costly as an M4 finder (as example). The parameters however 
are different and the price to pay for the possibility of several 
magnifications, more framelines, brighter finder and more clearly delineated 
rangefinder spot is this propensity to flare. One may argue that this is a 
high price and that would make sense. I am sure Leica designers are aware of 
the trade-offs and will not rest until a solution may be found eventually. The 
changes of the rangefinder construct during its lifetime from M3 to M6 0.58 
can be documented (I have an article in preparation for this topic) and I will 
argue that these changes are made to accomodate the additional requirements 
and not to cheapen the mechanism to save a few bucks. Such a conclusion is 
embarrasingly devoid of factual knowledge of the optical and engineering 
paramters..  
I will also argue that the current requirements (more functionality and less 
propensity to flare) can not yet be solved as it is a equation with too many 
unknowns. If any one knows about the stubborn resistance of the intricacies of 
rangefinder mechanism to all these demands, it are the designers themselves. 
Not to have a solution is not equal to not working on one. 
Compare the Hexar with the new 0.58 finder and you see that the designers of 
both companies had to make decisions. The Hexar has a  dimmer finder with less 
visibility of framelines and less propensity to flare. The M 0.58 has a very 
bright finder, excellent clarity of the rangefinder spot and clearly defined 
framelines and still somewhat more flare sensitivity, but less than the 0.72. 
Evidently none can square the circle as yet.
Another topic is the accuracy of the 0.58 finder. My updated website has new 
computations for the 0.58 and I can tell you that the 0.58 can be used with 
full confidence with the 1.4/50 and the 1/50 and the 2.8/90 and with good 
confidence with the 1.4/75 and 2/90. Of course when all conditions are in the 
negative,the 2/90 will be on the limit. But there is no calculation or 
practical experience that precludes the use of the 2/90 and all the others on 
a 0.58. In my view the 0.58 might be bringing the 0.72 to a marginal role. 
Please note that this text is part of an upcoming article and so is 
copyrighted and please when using part of this text, do not forget to quote 
and cite the origin.  
Erwin