Archived posting to the Leica Users Group, 1999/12/18

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Subject: [Leica] Split Prism Focusing
From: Bill Caldwell <sneeker@erols.com>
Date: Sat, 18 Dec 1999 07:34:34 -0500

Frank Dernie wrote:

SNIP --
I for one am fascinated. I knew that the prisms gave the rangefinder
effect of around 0,18 times focal length. Olympus and Nikon and probably
others made f4 focussing screens which increase accuracy to 0,2 x focal
length .... I had always rather assumed that SLR split images were WAY
less accurate than a Leica. My SLR has a plain ground glass, I prefer it
personally and assumed it to be more accurate for the reasons mentioned.
SNIP --

Similar to Frank Dernie, and probably many others, I also use a uniform
ground glass sereen, but with a grid division for focusing with my Rs. 
Why does the split prism focusing aid in the universal screen, black out
when using longer lenses?  

Sorry, this probably has been explained to me before.  However, in the
early fog of Saturday morning, I cannot remember why the split prism
focusing aid on the universal screen blacks out at longer focal lengths
(unless your eye is re-positioned at the eye cup correctly).  Mary,
where is that coffee?

Bill Caldwell
Northern Virginia
<sneeker@erols.com>