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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.
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