In a message dated 8/7/98 8:42:45 AM Eastern Daylight Time, cbarker@primus-
online.de writes:
<< believe that the microprism and split-screen focussing aids work by
having a fine tolerance to the angle at which they are viewed (through the
viewfinder). Therefore, when you move your eye, relative to the
viewfinder, you will find the difference in light transmission when there
is less than normal (i.e. with a slower lens) reaching the focussing
screen. Thus, I believe that you only have to move your eye slightly to
find the angle from which you can see through both sides of the split
screen - if it will be possible at all with the aperture at which you are
working. >>
My understanding is that because of the angle of the split-image, the image
you see on the split is derived from looking through the lens at an angle. If
the aperture is too small, the split image "sees" the diaphragm, and not
through the lens. The sharper the angle of the split, the larger aperture
(smaller f-number) that your line-of-site through the lens will begin to be
blocked by the diaphragm. The angle of your eye looking through the finder
gets added to the angle of the split (actually it's probably not added -
probably some trigonometric relationship, but my trigonometry is a little
rusty), this is why centering your eye is so important at increasingly smaller
apertures (larger f-numbers).
Microprism work similarly, they're just lots of very small angles...
Did anyone get this??? I thought I understood it until I tried reading my
explanation...
Paul Schings
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