Archived posting to the Leica Users Group, 2001/03/19
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>Excuse my naivete but I do not understand what Dick Gilcreast means when he
>comments in Volume 28 No 2 1995 on the SL that "the central microprism is
>cut for the relatively large lens speed of f2.8 ...and is therefore
>relatively more accurate shooting at f2". He also says that the best lenses
>to use with the SL screen are f2, f2.8, f3.4 and f3.5 lenses.
>
>He makes similar comments about the outer prisms being "cut for about f5.6".
>
>Could list members please help me decipher this ? I am new to the SL and
>require some assistance.
>
>
>Kind Regards,
>
>
>Tony Salce
>
>Kind Regards
Split image or microprism focussing aids for SLR's work best at
certain apertures. What this really is optimised for is the apparent
aperture at the film plane (or focussing screen), so for strong
retrofocus or telephoto designs this isn't exact. Retrofocus lenses
act like slower lenses, and telephoto lenses act like faster lenses.
In any case....
If a prism is cut to work best at f/2, then it will deliver the
optimal accuracy for f/2 lenses, but will be no more accurate for
f/1.2 lenses, so for these lenses the prism might not be quite
accurate enough. If a lens with an aperture smaller than f/2 is used,
part of the prism will go dark, so then the prism is totally useless
as a focussing aid.
Most SLR focussing aid prisms were cut to be optimal for lenses of
around f/3.5 or so aperture. Cameras with interchangeable screens
often had screens with microprisms or split image prisms that were
optimised for very fast or slow lenses, but fixed screen cameras had
to compromise, and whatever lenses you couldn't focus with the
central focussing aid you focussed on the surrounding ground glass.
Some cameras, like the SL I believe, and then some Minoltas and
others, used microprisms cut for slower lenses in the outer areas to
increase brightness, but if you tried to focus very slow lenses or
stopped down macro shots, most of the screen might go dark. If you
did this a lot, the factory usually had an option of changing the
screen to a ground glass type for a better compromise.
The central focussing aids were intended to extend the basic ground
glass focussing capabilities of SLR's in to the wideangle and
standard lens areas with a rangefinder (and the prisms act optically
exactly like a split image rangefinder) to be more competitive with
range/viewfinder cameras in practice.
- --
* Henning J. Wulff
/|\ Wulff Photography & Design
/###\ mailto:henningw@archiphoto.com
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