Richard F. Man wrote:
<< If
I don't use the dioptic (?sp) adjustment, I can still get a the focusing
screen "in-focus." But is the image really in focus in that case? >>
Yes, I think so, if you can see it's in focus at any time, with any
spectacles, or with or without viewfinder dioptric coeection, it's in focus.
The focusing screen doesn't move for a start and the image you are viewing is
in a sense projected onto its roughened surface by the lens and mirror.
The screen then re-scatters the light falling onto it whether it is in focus
at the screen or not and you can assume (I think) that you are viewing and
focusing on "the screen" when you look through the eyepiece and try to see
the image you have got.
Think of the focussing screen as a slide projection screen. You might need
specs to see it clearly, but if the image projected onto it is out of focus
in the first place, you can never see it in focus whther you have specs or
perfect eyesight.
[I guess you are thinking it's like looking through a microscope with
cross-hairs and wondering if you are going to get inaccurate focusing because
your eyes can strain to focus on an image that you are looking at even when
it is out of focus when it reaches the film. But I think that this doesn't
apply with a camera focusing screen.]
I guess that the purpose of the focusing screen is to prevent this sort of
ambiguity.
If the image is in focus at the screen then it is in focus at the film too.
Then it is "your problem" to view the screen! ;-) They are essentially two
separate issues.
Happy viewing!
Dave Bellamy.
http://members.aol.com/synthchap/
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