Re: [Leica] infinity lock on 50 cron collapsible

[SthRosner@aol.com]

Hello Rei:

The likelihood is that the infinity lock, that is, the knob which acts as a 
lock, was a carryforward from the 50/2 Summar lens of the 1930's. This lens 
did not have a grooved or serrated edge to use to grasp the lens mount and 
use it as a focussing ring. One focussed it only by using the knob. The 
infinity lock was just a means of locating the lens helicoids at their 
innermost focussing limit.

I don't remember for certain whether or not the 50/2 Summitar that intervened 
between the Summar and the first Rigid 50/2 Summicron has the grooves; my 
recollection is that rudimentary grooves were introduced with the Summitar to 
facilitate focussing without resort to the knob. And by the time we get to 
the Rigid Summicron, it has not only know and grooves but scallops cut into 
the focussing ring. 

Just a guess but I imagine that by the 50's when the Summitar was introduced, 
photographers had become so used to using the knob for focussing that Leitz 
retained it. It could not be employed on the Dual-range of course because of 
the complicated mechanical construction  of the focussing mount of that lens. 
And the knob disappeared completely on 50mm lenses in 1969 with the 
introduction of the first six-glass 50/2 Summicron.

Seth          LaK 9
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