Archived posting to the Leica Users Group, 2009/09/08

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Subject: [Leica] Leica RF patch size
From: Frank.Dernie at btinternet.com (Frank Dernie)
Date: Tue, 8 Sep 2009 20:39:18 +0100
References: <mailman.1622.1252365852.31809.lug@leica-users.org> <8CBFEC4608759C0-3E74-1129F@webmail-m079.sysops.aol.com>

I am not sure it is as much as that in most cases!
In SLRs with non-changeable screens the split image wedges and or  
microprisms are angled at equivalent of f5.6 so that all lenses  
available for the camera will be able to be focussed using these aids.  
Some interchangeable screen models have "fast lens" screens available  
where the prisms are angled to look out of a f4 aperture. This means  
the effective rangefinder base of a normal slr using a 50mm lens is  
about 9mm!
This means that the shortest lens for an SLR having the focussing  
precision of a 47mm base rangefinder (M9) in its split image spot or  
microprisms is 47x5.6= 263mm!!
Experience tells that SLRs focus fine, so either the M base is way  
more than needed, or the limit in focus precision is in the mechanical  
precision of the linkage for the rangefinder mechanism. It is  
automatically easier to get this precision on an SLR where there is a  
single optical element (the lens) and the rangefinder prisms are  
adjacent on the same part. The screen location precision is still  
important, of course.
Frank

On 8 Sep, 2009, at 18:53, lrzeitlin at aol.com wrote:

> . Accuracy of the split image RF is what makes its use possible in  
> SLRs where the base length is only the diameter of the lens used.



In reply to: Message from lrzeitlin at aol.com (lrzeitlin at aol.com) ([Leica] Leica RF patch size)