Archived posting to the Leica Users Group, 2000/04/19

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Subject: RE: [Leica] Question about minimum focus distance
From: Paul Chefurka <Paul_Chefurka@pmc-sierra.com>
Date: Wed, 19 Apr 2000 11:31:27 -0700

>-----Original Message-----
>From: Michel LE [mailto:le@ilog.fr]

>
>I've noticed that  the minimum focussing distance for 
>rangefinders (Hexar,
>Leica or Bessa)  is around 70cm, for a 35mm lens, while on 
>SLRs it is in
>the range of  30cmm or 40cm.
>
>Is this limitation induced by parallax error involved with rangefinder
>mechanism or is there any optical justification for this ?

AFAIK it's mainly a parallax issue.  Nothing would prevent designers from
increasing the length of the helicoid, but the parallax would make framing
pretty hit-or-miss at 30cm.

It also gets harder to focus precisely, due to the trigonometry of the
system - there's a much better chance the system isn't focussing where you
think it is.  The subject can easily fall outside the depth of field, which
at close distances in narrow to begin with - so the proportion of
out-of-focus images will skyrocket.

Paul Chefurka