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Re: [OM] tripod mounting 180/2.8 + Oly 2X?

Subject: Re: [OM] tripod mounting 180/2.8 + Oly 2X?
From: "John A. Lind" <jlind@xxxxxxxxxxxx>
Date: Thu, 16 May 2002 23:51:26 -0500
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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