From: nitram@tellabs.com (Marty Hewes)
Newsgroups: rec.photo
Subject: Re: Rangefinder calibration
Summary: What about moving the lens assembly?
Date: 28 Jul 92 13:17:59 GMT
Distribution: na
Organization: Tellabs, Inc.

In article <fred-mckenzie-270792093632@128.159.169.79>, fred-mckenzie@ksc.nasa.gov (Fred McKenzie) writes:
> In article <4603@daily-planet.concordia.ca>, honna@concour.cs.concordia.ca
> (SEGEL honna) wrote:
> >  I want to re-calibrate the range-finder.  I'm not shy about the surgery.
> > I need to know a few things about range-finders generally...
> 
> I recommend setting the focus so that the images coincide at infinity, with
> the end of lens rotation also set at infinity.  At that point, the
> resulting infinity image at the film plane should be sharp, using ground
> glass and a magnifier.  Be sure that the ground side of the glass is facing
> the lens.  Otherwise, the image won't be at the film plane.
> 
> At other distances, the accuracy of the rangefinder depends on the shape of
> a cam, in cameras I'm familiar with.  There may always be a slight
> discrepancy between the distance scale and actual focus.  However, actual
> focus should agree with coincidence of the rangefinder images, at all
> distances, once infinity is set.
> 
> fred-mckenzie@ksc.nasa.gov

A related question.  I have a number of cameras that have lens/shutter
assemblies mounted on bellows or at the end of a tube (a Kodak Pony I was
playing with).  I have noticed that the lens/shutter assembly is often
shimmed out from the lens mount with heavy paper (on the Pony, an Ikonta,
and a couple Agfa folders I have).  I tried to calibrate the focus scale on
the Pony by placing ground glass at the film plane and adjusting the
(independantly adjustable) focussing scale on the rotating front element to
the point where the image is sharp at infinity and the scale reads infinity,
much as you have suggested.  I found that the focussing scale was off by 
a significant amount when refocussed to the near end.  Recalibrating the
near end caused a similar error at the far end.  

I am left wondering if the lens to film distance affects not only the 
absolute setting of the distance scale, but also the range, or length
of the scale (the distance on the scale itself between the near and far 
markings).  Note that I am playing with front element focussing lens
assemblies here.  Does anybody know if increasing the lens to film distance
is likely to lengthen or shorten the scale?

In case anybody is wondering why I would bother with Kodak Ponys...
I sawed opposite ends off two of them, epoxied them end to end, and made a
pretty fair full frame stereo camera.  Firing both shutters at the same
time is tough though, even with a dual cable release.

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