"Jean-David Beyer" <jeandavid8@[EMAIL PROTECTED]
> wrote in message
news:uujVj.930$%X1.495@[EMAIL PROTECTED]
> 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.
> /V\ PGP-Key: 9A2FC99A Registered Machine
> 241939.
> /( )\ Shrewsbury, New Jersey http://counter.li.org
> ^^-^^ 11:35:01 up 3 days, 4:07, 3 users, load average:
> 4.16, 4.13, 4.08
I think you may be right that its the screens which are
made at an angle rather than the aperture, its been too long
since I learned about this stuff and the books are not
handy.
Soft dot edges have always been a problem. Monckhoven's
intensifier was intended to remedy this. It has the peculiar
property of being both an intensifier and reducer because
the cyanide will dissolve the low density silver before the
intensifier works so the net result is to increase the
contrast of the edges of the dots.
--
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@[EMAIL PROTECTED]
|
