Looking at various SLRs, I see some have the option of replacing the focus screen. Nikon, for example, has one called a Katzeye or something like that.
I know some SLR's i believe the Pentax K10D you can not see the AF points through the viewfinder. They have an overlay that can be put in the viewfinder so you can see the points. Is this what your talking about.
The 5D has these. You can get one that has grid lines for horizontal and vertical alignment. They have one you use just for manual focusing. The one that comes with the camera has a matt finish on it.
scroll down this list and you will see all the different types Canon has.
It's a small piece of "ground glass" (actually they're precision machined , but essentially the same thing) that the lens focuses the image on. Think of it as the screen in a rear projection video system. The lens essentially projects the image onto it and when you look through the finder, that's what you see.
Different screens have different markings and functions. There are split prisms, matte prisms, grid prisms, and, if you want, you can have custom screens built. Beattie used to make them for film cameras, but I dunno if they're still around
It is good news to learn that microprism and split image focusing is FINALLY coming to digital SLRs. Without them manual focus with digital SLRs is still very difficult, particularly under low light level conditions where autofocus has problems anyway.
Personally, I prefer to do my own focusing rather than let the camera decide it for me. It makes mistakes.
In my opinion one of the great losses brought on by autofocus lenses durng the rise of digital SLRs is the loss of manual focusing aids within the viewfinder. At one time that was a primary feature of SLR cameras. It is nice to see that coming back.
