ERROR CORRECTION Re: [Leica] Focusing difficulties

[Ken Iisaka <kiisaka@pacbell.net>]

Ok, I goofed.

The eyepiece in a pentaprism finder does indeed have a magnification of
about 4-5x, however, the "magnification" does a comparison at a viewing
distance of 25cm or so (I am afraid I forgot the exact figure.  I will have
to look it up.)  In effect, because the eye would be much closer to than
that to the focussing, so the effective rangefinder base length is only the
focal length of the lens times 0.18 times 2.  So the effective base length
with a 50mm lens is about 18mm or so, hardly accurate enough for a fast
lens.

For my Nikon F3T camera, I maintain a collection of focussing screens for
different purposes: the type E, a groundglass with a grid is what I use most
often.  For available light, I use G2 which is microprism throughout, though
much coarser than SL.  I occasionally use type R which is similar to type E,
but with a split image prism in the middle.  Nikon cautions that the prism
on this screen looks at even narrower points, so it advises against using
for a lens faster than f2.8.

From: 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