In article <5rqlhj$418$1@trotsky.cig.mot.com>, manuelg@comm.mot.com says...

>With all the interest in brighter focusing screens, I took
>a look at mine to see if I should get one. I have an F3HP
>and noticed something peculiar about my split prism focusing
>aid (K screen).
>
>1. When I have a lens that is f4 or smaller with a UV filter,
>the split prism focusing aid becomes unusable.
>
>2. When I have a lens that is f2.8 with a UV filter, and stop
>down to f4, the split prism focusing aid becomes unusable.
>
>3. When I have the same f2.8 lens with a 2-stop circular
>polarizing filter which effectively makes the lens an f5.6,
>the split prism focusing aid is fine.
>
>Can anybody explain this to me?

The polarizer (or most any other filter) cuts light, not
aperture... BTW, the HP finder allows considerable shifting
of sight line from dead center - where even an f5.6 lens should
be useable with the rangefinder, uh, "focusing aid" (I hate
those things...;-)
Hope This Helps
(David Ruether - http://www.fcinet.com/ruether )