Archived posting to the Leica Users Group, 1999/01/12
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]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 glass. 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 haige@compuserve.com