At 18:46 5/16/02, Moose wrote:
In addition to the f-stop, it hs to do with the angle of the wedges and
the angle of the light coming in from the lens. I have no idea what's
being done today, but back in the Nikon F & F2 days, Nikon made split
image focusing screens for tele (and WA?) lenses in addition to the
regular ones.
Moose
The "split image" rangefinder on the focusing screen works by slanting the
surfaces of the two halves in opposite directions creating prisms out of
them. Since the surface of them is smooth, the image in the split circle
does not appear to be in or out of focus as it does on the matte
area. IOW, the image in the circle is aerial, and is not formed on the screen.
The rangefinder split works by one half using ray paths from one half of
the lens and the other half using ray paths from the other half of the
lens. When out of focus, the ray paths at their boundary end up separated
causing the image in the circle to diverge. When in focus, the ray paths
at their boundary converge and the two halves of the split image converge.
Since the split image only uses half the lens' ray paths in each half of
the circle, it goes "black" at narrow apertures.
BTW, the "microprism" ring around the split image is formed of tiny
versions of same type of prisms used in the split image. As the aperture
is narrowed, the split image will go black first. As the aperture is
narrowed further, the microprism ring will eventually "go black"
also. Usually this isn't encountered except in very low light.
-- John
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