[pure-silver] Re: History... Clarification
- From: Jean-David Beyer <jeandavid8@xxxxxxxxxxx>
- To: pure-silver@xxxxxxxxxxxxx
- Date: Tue, 20 Oct 2009 07:16:10 -0400
Richard Knoppow wrote:
----- 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 Componon-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.
Actually, it was the half-tone screens that are rotated to avoid moire.
Lozenge-shaped apertures were used to diddle the curve shape. Angling the
half-tone screens for color had an interesting non-technical aspect.
F.E.Ives angled the screens in 1881 and did not patent the idea, as it was
obvious to someone skilled in the art (and therefore unpatentable). 10 years
later, someone patented the idea and Ives was threatened with a lawsuit for
infringing the patent. He had to prove that he had used the technique a
decade earlier and that therefore the patent was invalid.
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.
Done right is the critical point. The reason 50-line screens were used in
newspaper printing is that the paper used was so very bad that the screen
clogged up with lint with finer screens. The paper used was later improved
somewhat, so that 65-line screens could be used. In book printing where the
intent was to get good images, 120-line and 133-line screens were used, and
once in a while, 200-line screens were used, probably only for photo-offset
printing.
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.
Actually, F.E.Ives was offended by this. His view, and it seems to be the
case, is that if you understand the process, you could control the contrast
by adjusting the screen height (the height of the screen from the film), and
this is certainly the case. But most photo-engravers did not understand the
process, and so resorted to all manner of high-cost tricks to cover their
ignorance.
I have found that with problem negatives, a second exposure with the lens
stopped down considerably can be used to burn the highlights. In theory, you
could do even more, but I have never found it necessary.
Now note that 3/4 of the light is lost because only 25% of the screen is
open. And lith film is kind-of slow anyway. So my exposures are of the order
of a minute with the screen height I usually use. This with an Aristo D-II
HI cold light head in the enlarger. I do not need this much exposure to
print a normal image on paper.
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.
Actually, there are two parts to the dot-gain problem as I see it. The first
is to fix the problem that even with lith film properly developed, the first
generation screened image is composed of soft dots. Now if you contact that
to another sheet of lith film, you get hard dots, but the % dot area
changes. This can be pretty well handled by adjusting the exposure of the
contact printing. In early days, they did not know what to do about soft
dots. F.E.Ives invented some intensifiers to minimize the effect, and these
days, with litho plates such intensifiers are less necessary.
The second is the spread of the paper.
There was also often a great deal of hand work
done, especially for the best quality reproduction.
Actually, F.E.Ives loathed hand work and never did it with his plates. He
felt the hand work spoiled the quality of the image as it was usually
obvious to a trained observer. His view was that the hand work was needed
only to compensate for the ignorance of the original plate maker. When
people cared about quality, they paid higher prices for Ives' plates than
the competition, because the fine-etching and other techniques actually made
the cost higher.
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.
I do not know either.
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.
You are off by a generation. We are speaking of my grandfather and
great-grandfather. My father taught French at the University of Buffalo and
took a lot of color slides of flowers, etc. ;-)
--
.~. Jean-David Beyer Registered Linux User 85642.
/V\ PGP-Key: 9A2FC99A Registered Machine 241939.
/( )\ Shrewsbury, New Jersey http://counter.li.org
^^-^^ 06:45:01 up 19 days, 15:44, 3 users, load average: 4.20, 4.35, 4.23
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