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Re: [OM] Mirror lens contrast - was Four new pictures

Subject: Re: [OM] Mirror lens contrast - was Four new pictures
From: Matthias Wilke <Matthias.K.Wilke@xxxxxxxxxxxxxxxxxx>
Date: Fri, 1 Jan 1999 01:05:52 +0200
John wrote:
>My understanding is the central obstruction in a visual design always lowers
>contrast, regardless of the contrast of the subject.  My own personal
>experience bears this out.  High contrast subjects tend to look a little
>flat, low contrast subjects are just plain horrible.
>
>There are multi-mirror designs optimized for photographic only use with very
>large central obstructions, around 500f the clear aperature area.  These
>astrographs are not for visual use, but are capable of exceptionally fast
>f/ratios (< f/4) and outstanding contrast, rivaling that of a refractor.  Of
>course, they are also sinfully expensive .....

I think the Rubinars are in spite of the fact that there are astronomical
versions around, optimized for photography. For example, the focusing
happens through rotating the front element and further they go to macro
till 1:4. Pure astronomical lenses are optimized for infinity and are
absolutely useless for terrestrical photography. I have tried years ago the
Vixen (?) 90mm diameter spotting scope (also of the Maksutov type) and the
test pictures of terrestrical objects showed very, very low contrast and a
flat definition. But this happens with refractors optimized for infinity,
also.

Matthias



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