In article <4l502n@news-ipg.umds.ac.uk>, A.D.Smith@umds.ac.uk says...

>On the subject of focussing a 35mm SLR while being short-sighted.
>I always understood the principle to be that the camera lens formed
>a focussed image on the ground glass viewing screen. Being short >sighted I can easily see (and focus clearly on) a ground glass screen >at a couple of cm from my eye. So why should my eyslglasses make a >difference to my ability to tell whether the image is focussed or not. >As I see it, all I'm actually looking at is an image on a screen
>about 2cm from my eye, which I can focus on without my glasses. Is
>this true, or am I missing something. [....]

You are right that you are viewing and focusing the lens image as
it is projected onto the viewing screen, and your ability to do
so depends only on your ability to see the ground surface of that
screen sharply (though camera manufacturers add changes to that
surface, and add additional optics to that surface to brighten the screen image, and some of those additions may interfere with sharp,
contrasty viewing of that surface, unfortunately [thus making AF
appear necessary to many people...;-]). The viewfinder optics give
an apparent viewing distance that is far greater than the actual distances of a few inches away from the eye so that most (younger)
folks can see the viewing screen easily. As we age, our ability to
focus over a long range diminishes, and we may no longer be able to
see sharply objects (or viewing screens) at some distances without
the aid of corrective lenses, either fitted to the camera eyepiece,
or to us (glasses, contact lenses). The former option does not work
if correction is needed for astigmatism in addition to distance
(unless the photographer only shoots vertical, or horizontal, photographs).
Hope This Helps