Richard Knoppow wrote:
> It appears that the shape of the iris affects out of focus areas of the
> image. This is perhaps part of the effect called bokeh by the Japanese.
> In any case bright points which are not sharply focused are rendered in
> the shape of the iris. This may not be as noticable for enlarging where
a
> flat surface is imaged onto another flat surface.
> In making half-tone plates the iris is imaged by the half tone screen as
> an array of spots or dots. By using a square aperture the intestices of
> the dots are at the corners so the variation is smoother.
Right, although I find that a round hole is not too bad. But I do halftone
starting with a continuous tone negative (a positive print or other
positive
flat art is usually used) and make the half-tone on OrthoLith. These dots
have soft edges (undesirable) so I contact print them onto another piece
of
OrthoLith to get a suitable half-tone negative.
> For color work each of the images is photographed using an iris with a
> lozenge-shaped aperture at a different angle.
That might be so, but I never heard that.
> I can't remember now if this is to prevent moir[e] but I think it is.
When my great grandfather (F. E. Ives) devised the process, he angled the
half-tone screen to prevent moire, but AFAIK did not use lozenge shaped
diaphragms.
> In any case there is an optimum set of angles for the apertures.
This is true for the angles of the half-tone screens as well.
In fact an amusing thing happened to F.E.Ives. When he first made color
half-tones, he naturally angled the screens between the different colors.
He
did not bother to patent that because it was obvious. Years later someone
else patented the idea of angling the screens and sued Ives. Ives had to
go
to court at considerable expense to prove that the patent was invalid both
because it was obvious (kiss of death for patents) and because of over a
decade of prior use.
> These apertures are usually in the form of Waterhouse stops and is the
> reason process lenses usually have a slot in the side.
In any case a slot is needed to set the aperture (square or not) to the
correct angle. This is all about sealed glass half-tone screens, not the
plastic contact screens more recently used. Of course, with a round
aperture, none of this makes any difference.
> My barrel mounted Apo-Artars have the slot but the shutter mounted one
> does not. The barrel mounted Artars also have 20 blade irises and a very
> nearly perfectly round hole.
>
The 150 mm Componon-S 5-blade iris does not make a geometric pentagon.
What
would be straight edges are actually semi-circular convex to the edge of
the
lens. I.e., at the "corners" of the iris the diameter is larger than in
the
middle of what would be a straight line. In other words, the corners are
farther from the center than would be expected if a true pentagon were
used.
This clearly because the iris blades are made that way. It would be
perfectly easy to make them straight if Schneider had wanted to. I assume
this was done for optical reasons and not to make the mechanics cheaper.
--
.~. Jean-David Beyer Registered Linux User 85642.
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