Archived posting to the Leica Users Group, 1998/06/04

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Subject: Re: [Leica] LUG -- what's the point?
From: Paul and Paula Butzi <>
Date: Thu, 04 Jun 1998 13:34:42 -0700

At 12:54 PM 6/4/98 , Harrison McClary wrote:

>If you want to check the focusing accuracy of your range finder the only 
>way I know is to focus with the camera on a tripod on something then flip 
>open the back and put some vellum over the focal plane and see if it is 
>in focus there.

If I were going to try this (and I'm not) 
I'd worry a lot about the following:
1. Depth of focus will be really minimal 
    with the wide apertures available on
    Leica lenses.  Rather than use vellum, 
    I'd use something that I could ensure
    would be FLAT, like a small piece of 
    ground glass that would rest against
    the film rails.
2. even so, it would be difficult to get the 
    ground glass ground surface exactly
    registered where the pressure plate 
    and the film rails would put the front
    surface of a piece of film. Film flexes.  
    I assume that Leica take this into
    account when building cameras.
3. Very small errors in focus would be 
    very hard to detect on the GG but might
    be quite apparent on film.  At the very 
    least, you'd need to check the focus
    with a loupe.  My preference would be 
    to focus on the GG with a fairly high
    power loupe, then check the rangefinder, 
    rather than the other way 'round.
    Looking at the GG with your naked eye 
    would be pretty much useless.
4.  Absolute accuracy probably requires 
    focusing a microscope on the *aerial*
    image formed at the film plane, since 
    even a fairly high power loupe on a
    ground glass will not allow you to 
    resolve all the detail in the aerial image.
5.  All of this fooling around at the film 
    plane would make me nervous if the
    cable release let go and let the shutter 

If I really had some reason to believe 
that the rangefinder was out of whack,
I'd stretch out a tape measure for 30 
feet or so, and then expose multiple frames
of various marks on the tape, with the 
aperture wide open.  I'd do it on
really fine grained film like Tech pan, 
or maybe Ektar 25, and then I'd
examine the image of the tape measure 
on the negative with a low-power
microscope.  Focus each frame separately.  
Remember when evaluating the actual 
plane of focus that the depth of field is 
asymmetric about the plane of focus - 
one third in front, two thirds behind.  
So if you focused on the ten foot mark, 
you'd expect that the marks at 9 feet and 12 feet
would be equally out of focus, with the 
ten foot mark being sharpest of all. 

All of that sounds like a lot of work to me.  
In the end, I'd probably just send the camera 
off and have it checked/adjusted.  All right, I confess.
Maybe I'd buy another M6, and send the 
suspect one off to be checked/ adjusted.  
That way, I'd not have to go without one.

- -Paul