SLR AF systems don't see the focus screen, but much light is abducted by passing THROUGH the main mirror into a cavity in the bottom of the camera where it is tortured and expires.
Edmund
Yes, AFAIK there is a secondary, small mirror used to work af, the poor positioning of which causes the problems of front or rear focussing.
But isn't it also something to do with light travelling backwards through the eyepiece and screwing exposure somewhat? If the screen type makes no difference to that problem, then why don't they simply use old-type screens and thus, even if without a split-image device, manual focus would be more accurate as long as the screen was properly mounted?
I never use any af point other than the central one; that is seldom the main locus of my subject, but as I use only that one central area, a split-image would serve me just as well for the times it is the main area of interest, and be very useful with my non-af optics as mentioned above. A focus-confirmation light is not a convenient alternative because it lies outwith the image, on my cameras, at least.
Using alternative focusing areas of the af system would slow me down a lot. I don't use tripods anymore, and I can't think trying to change from one af zone to another is a rapid way to work. Yes, in some cases zone focussing is perfectly good enough. That said, I supose one could pre-set an af system so as to force one to make a series of shots with the main subject in a set position: a series of faces all at the magical third, for example...
