Archived posting to the Leica Users Group, 1999/01/12

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Subject: [Leica] Ground glass focusing check on M cameras
From: Harry Haige <>
Date: Tue, 12 Jan 1999 08:38:39 -0500

Hello Luggers,

I've been a die-hard user of the M system for amateur and
technical/scientific photography for many years and offer the
following remarks in light of recent postings related to checking
of the focus of the M cameras with a ground glass. 

How to do it: You need a piece of thin ground glass 35-36mm high
and 40-50mm wide.  Ground glass is readily available and easily
cut by scoring the plain side.  The edges and corners can be
sanded to make them smooth.  The height is important since the
glass must fit ATOP the inner two polished rails and WITHIN the
outer two polished rails on the back of the camera.  Easiest way
to clean it prior to use is gentle scrubbing with a soft
toothbrush and mild hand soap, rinsing well with PURE water.  Let
it air dry, do not wipe. 

Remove the bottom plate of the camera, remove the rear door, and
reconnect the bottom plate.  Cock the shutter, set the exposure
to B, and hold the shutter open using a locking cable release. 
Then install the ground glass, ground side toward the lens, and
secure the glass in place.  One way is to do this is with 4 SOFT
wooden triangular cross-section tooth picks, like Stim-U-Dents
(r), the tips gently wedged between the glass and the door frame,
holding the glass firmly against the rail.  There should be
absolutely no movement of the glass when pressed from either side.

How to do it the right way:  The problem with the above is that it
is really not possible to focus critically on a ground glass, even
with the commonly used 4-5X magnifiers.  That is why Leitz
included on e.g. the Focoslide a clear central patch with
reference cross hairs or "crows feet" to permit focusing
critically of the aerial image using a high power magnifier of
30X.  An unused Focoslide screen could be cut to size for the
purpose, or alternatively you can add a clear patch to the center
and maybe also an edge of the 35mm image area on your ground

To do this you need a small cover glass that is used with a
microscope slide.  Draw as fine and black an X as possible at the
center of the cover glass. Place a small drop of Canada balsam on
the X, and then press the cover glass firmly onto the desired
spot of the ground side of the ground glass.  The Canada balsam
will spread to make a clear spot the size of the cover glass with
the focusing cross hairs in the center.  Let it dry before use. 

When used with a high power magnifier, this setup will reveal the
full strengths and weaknesses of the focusing accuracy and lens
quality.  For instance, you will be pleased to be able to read
the fine print in a newspaper at 15 feet with a 50mm lens and
surprised to see that (unless it is an APO) your lens clearly
cannot focus red and blue targets at the same distance setting.
No telling what your comparison of the rangefinder focusing
indication with the actual image focus at the film plane reveals.
There are many variables involved so don't be surprised!   

Harry Haige