I have heard it said (maybe in a user manual?) that the split collar microprism is used in three ways
1) the matte surface covering the entire screen for initial focus / telephoto focus
2) the collar/ring for close shots (say less than 10 feet)
3) split prism for exact detail or very close shots
This seems to work for me, not sure how accurate it is.
Jamey
---------- Post added 08-12-14 at 08:49 AM ----------
ok it was the LX manual...
"The Pentax LX comes equipped with the standard SC-21 split-image/microprism focusing screen which provides a split-image center spot surrounded by a microprism collar on a matte field The SC-21 is fully interchangeable with eight other focusing screen models to provide focusing versatility capable of meeting almost any picture-taking need (see page 49). When using the split-image center spot simply turn the lens focusing ring until the broken image in the center spot in the viewfinder aligns as one.
When using the microprism collar surrounding the center spot, rotate the lens focusing ring until the "shimmering" effect in the collar area seems to d disappear.
The matte field is quite handy for focusing in rapidly changing conditions where there simply isn't time to use either the split-image spot or microprism collar or for focusing with long telephoto lenses having small apertures. To focus, turn the focusing ring on the lens until the image on the matte field appears sharp and crisp. "
Pentax LX