The problem here I believe can be two things:
1. Refractive error of the photographer. However the use of microprism/split
prism to focus eliminates this in most cases.
2. When an image is focussed, we are actually appreciating the focus on the
focussing screen that is designed to be at the same distance as the film.
Therefore if your focussing screen is slightly out (frequent screen changes
can warp the frame for holding the screen in place) or more often the film
is not flush with the pressure plate then the small differences you see can
be explained. The problem obviously occurs when the lens is wide open and
the depth of field is unable to cover for the minute film to focus screen
distance differences. The problem is "real" enough that the Contax RTS III
when released showed it has has a suction device to smooth out these film
plane variations as a source of focussing error. The films age could also
play a part perhaps tending to bow out in the centre etc.
Just some suggestions.
Oben
I have thought about this too. If you have a refractive error due to
poor vision then it seems to me that you will have a blurred image
and will focus for the best blur, probably giving you a range in
which you turn the focus ring and not much happens. I just tried it
with my OM4T by defocussing the viewfinder. It was certainly
difficult to get focus that was accurate within 2 or 3 inches. It
was more difficult with the ground glass than with the split image or
microprism. Although in both cases they seem confident of the
accuracy of their focus which probably would not happen if they had
vision problems.
I wondered whether the focus screen was upside down, but I don't
think you can put it in wrong. Can you?
I like the old, bowed film idea.
It seems like John Hermanson said something one time regarding
inaccuracy of mirror position causing focussing errors. I don't
remember the context or what caused it.
Winsor
--
Winsor Crosby
Long Beach, California
mailto:wincros@xxxxxxxxxxx
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