Ok, I'm gonna step out on a ledge and show my ignorance. What is a viewing screen? I've been reading threads and I seem to gather it's something to help you focus when using a manual focus lens?
The viewing screen is the thing you're looking at when you look into the viewfinder of the camera. It's a rough-surfaced glass/plastic panel that the lens projects the image on (like a rear-projection TV). Different screens have different properties that make them better/worse for manual focusing; you typically trade off brightness at small apertures for accurate image rendering (allowing accurate focus by eye) at large apertures.
I see. From what I read it sounded like it was something that people buy to add on to the camera to make focusing easier. I guess that is not the case then?
People do buy different focusing screens for their cameras to replace the camera's original screens. Most DSLRs come with screens that are optimized to give bright images on smaller aperture lenses (like the f4-5.6 kit lens zooms that often come with the camera), by directing more of the light coming from the center of the lens towards the viewer's eye. These screens, however, don't take full advantage of the light from large-aperture fast primes --- the light coming from the edges of a wide-open fast lens gets lost. The normal focusing screen essentially stops down every lens to ~f2.8, so you can't see and focus the actual image formed by, e.g., an f1.4 lens. Replacement screens (like the EEs for the Canon 5D) aren't as efficient at directing light from the center of the lens towards the viewer (so they are darker on slower lenses), but they don't lose the light from the edges, so you can see the actual image that will be formed on the focal plane (makes focusing easier). Some people also buy third-party focusing screens with additional focusing aids (like microprisms or a split-prism) built in, like the focusing screens on older manual-focus SLRs.
