Andreas Helke posted a reply:
In a DSLR about half the light goes to the viewfinder. The other half goes to an extra mirror and from there to the autofocus sensors in the bottom of your mirror chamber. The part that is used for the autofocus sensors never reaches the focusing screen.
In a split screen focusing help you either see just black if the aperture of your lens is too small or two out of alignment parts of your image that get into alignment once you reach correct focus. Such a replacement screen is very useful in a xxD camera where you keep the focus lights. I don't like it in the xxxD series where you loose the focus lights with a Haoda or Katz Eye screen.
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