Leica Calibrating lens focus with fresnel screen


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OK....I am going to cut a screen to use in the film slot of my M's to calibrate the focus of various lenses. My question is how large to cut the screen? Obviously the length of the picture area is 36 mm but that's not an issue because the M cutout is more than 36 mm, that is if you place the glass below the rails or even on top. the height is supposed to be 24mm although the width of film with perforations is 35.3mm or so.

So where does the fresnesl screen go....on top of the rails or inbetween? The screen is obviously thicker than film. furthermore , after check my m4 and M5 there are different sized insets for the rails. Just wondering how to make it fit. Seems like it should be easy to cut a piece of glass or plastic for a focus screen and just tape it on the rails somehow.

Also, would it be helpful if there was a central high performance focusing prism in the center?
I know that Kim Coxon does this- my way is different.

I Use a blank strip of 35mm negative with scotch tape over it, and use a 15x loup with a glass plate at the point of focus to hold the negative flat.


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What do you think of a 35mm square fresnel and a split image microprism in the center? It would not cover the entire negative size length, but maybe that is not necessary. It would , however, allow one to reorient the "glass" to change the split prism to the type of object in focus....and also probably allow use of a less highly magnified loupe. My BLl watchmakers loupe is 4x and it could not press against the glass and focus like a photographers' loupe on a LF cam.

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