Focus screen thickness: 75mm vs 80mm

Rolleicords with non-interchangeable focus screens have a size of 56 mm x 62.5 mm in vertical orientation and Rolleiflex SL66 have a size of 55.5 mm x 62.25 mm in horizontal orientation.

Will focal length have any effect on thickness of focus screens?

http://rolleiflex.us/blogs/news-tec...-sizes-for-all-rolleiflex-slr-and-tlr-cameras

No. But yes...

A glass plane, and that is a ground screen too (I leave the fresnel-lens aside now), has a certain effect on focussing.
However that is independant on focal length.

The effect the thickness of the screen, is to be taken care of by positioning the ground plane (shimming, depending of orientation of the screen).

Ground screens of different thicknesses cannot be interchanged if the arrest is on the smooth side.
At least in theory... We are typically speaking of small thicknesses and even smaller differences in thickness.

the image is focused on the bottom of the screen, the ground side facing the mirror or lens. as long as that surface is focused to agree with the film plane, the thickness has no bearing on the image... thats just extra clear glass.

If you're replacing the screen it's worth looking at the Chinese Fresnel split focus screens available on Rbay. I bought one q few years ago for my Yashicamat 124 but it sat in a drawer until I eventually fitted it to my Microcord (a British made Rolleicord) it's now a lot brighter and easier to use.

Ian