[pure-silver] Re: History... Clarification
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <pure-silver@xxxxxxxxxxxxx>
- Date: Mon, 19 Oct 2009 22:31:09 -0700
----- Original Message -----
From: "Ray Rogers" <earthsoda@xxxxxxxxx>
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Monday, October 19, 2009 4:02 PM
Subject: [pure-silver] Re: History... Clarification
--- On Mon, 10/19/09, Ray asked:
> What difference does round vs square apetures make?
& Jean-David Beyer wrote:
Well, I use a round aperture because that is what my
Compon-S f/5.6 180mm lens has. But square apertures are
better (the H.D curve has a better shape). People also
used
other shapes for different reasons.
??? Just how many... and what designs for apertures are
there?!
I have never ventured to look past the more or less
circular (hexagonal? I never bothered to count)
apertures... [come to thnk of it, when you say "circular"
does that mean circular or approximatly so?]
I recall during the last solar eclipse, there was
discussion of how the circular sun could be imaged by all
shapes of "apertures" including the spaces between tree
leaves....
Were any of these non-circular-ish apertures found on
common (pictorial) lenses?
Ray
Most process lenses had means for using Waterhouse
stops. These are sheet metal stops which are inserted in a
slot in the lens barrel. Square stops produce better
half-tone tone reproduction because the corners touch when
the dots become large so the coverage can be better than is
possible with round stops. For four-color work the stops are
lozenge shaped and are at very specific angles to control
coverage of the ink and to avoid interference patterns known
as moire.
A google search will find some additional information
but Jean-David is by of being the horse's mouth here.
Half-tone printing using screens is a quite old
technology. While many identify it with poor quality
newspaper printing it was capable of superb quality when
done right and printed on decent paper.
It is not a trivial process: the exposures (sometimes
several) are important in controlling the curve of the
reproduction and the whole process from photographing to
plate making must be done with skill and care. Some
termimology current in digital plate making has come down
from the original process such as dot gain. This is the
amount the dots spread out on the paper due to diffusion of
the ink and must be compensated for in the plate making
process. There was also often a great deal of hand work
done, especially for the best quality reproduction. There is
a great deal more to this and perhaps Jean-David may be
prevailed upon to expand on it.
As far as personal stuff on the list I have no idea
what this is about. Sometimes one accidently posts quite
personal material intended to be private or something far
off topic but I found Jean-David's biographical information
fascinating. Of course, we are old friends from Usenet
(where we both still follow the Darkroom group). Both his
father and grandfather contributed very substantially to the
printing trades and to photography and we should be glad to
have such a direct and reliable source among us. I have
certainly not seen anything objectionable.
--
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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