Art wrote:
>I have a question about the 1-12 screen. It is always described as a
>cross hair on a clear field but if you look closely the field is not
>clear. The field is actually a fresnel lens pattern. The central
>circular region that contains the cross hairs is indeed clear but the
>rest of the field is made up of fine concentric circles.
All focusing screens, matted or not (but probably with the exception of
the endoscopic 1-9?), have a fresnel lens (concentric circles) in the
"matte" area. This is the base surface that forms the image and is used
for focusing. The matte area that exists on most screens lays on top of
this lens; it facilitates (fastens) focusing and enables you to check
the depth of field, but also darkens the viewfinder; that's why the
1-12, intended for larger-than-lifesize (macro) magnifications has a
clear screen (other "clear field" screens are the 1-5, 1-6 and 1-7;
these have microprisms to assist focusing).
Even with the 20/3.5 head on the bellows at max. extension (12:1 macro)
you will see an image using the 1-12, while a matted screen (including
the "2" series) would be completely dark (not to mention any additional
focusing aids [split image,microprisms]).
You're supposed to focus on the fresnel lens, using the cross-hairs to
fix your eyes on the surface of the screen (and also to calibrate the
viewfinder).
The 1-11 is for "normal" macro work; it enables you to check the depth
of field because it is matted.
Hans
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