Archived posting to the Leica Users Group, 1999/10/03

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Subject: Re: [Leica] Electronic rangefinder ?
From: Godfrey DiGiorgi <ramarren@bayarea.net>
Date: Sun, 3 Oct 1999 23:47:36 -0700

>> The Contax has, as you know, an electronic rather than mechanical
>> rangefinder.
>
>How does an electronic rangefinder work?

There's a discussion of electronic AF mechanisms (electronic 
rangefinders) in Norman Goldberg's book "Camera Technology - The Dark 
Side of the Lens". He explains it much better than I can. 

There are a couple of different kinds - moving prisms/mirrors with active 
IR projection and passive receptors are the two most popularly used 
types, and they're sometimes combined for even more accuracy in a wider 
range of illuminations. Both types use a comparator circuit: it 
essentially turns the received light into a frequency histogram which is 
used for matching until the best match is achieved. The AF used in SLRs 
is related to this technology, the sensor technology is different (more 
compact and designed to operate behind the main lens rather than in 
independent sensor windows) but the comparator principles are closely 
related. 

Godfrey