[Leica] Focusing difficulties

[Frank Dernie <FrankDernie@compuserve.com>]

Ken Iisaka wrote

<The split prism screen is indeed rangefinder: the image you see in these
two
>halves are aerial images from different points of the lens.  Typically,
the
>angle of the prisms are so that you see the image at the points at f5.6 or
>so, or 5 degrees or so off axis.  This means that the split prism
>rangefinder has a base length of (focal length) x 0,18.  With a 50mm lens,
>the base length is 9mm.  Given that the viewfinder has a magnification of
>about 4x, the effective base length is 36mm.


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,
OTOH I did not know that typical viewfinders have a magnification of 4. 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.

thanks for the info
Frank