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nobody15

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Since: Oct 31, 2004
Posts: 906



(Msg. 16) Posted: Tue Feb 22, 2005 11:05 am
Post subject: Re: Square pinhole [Login to view extended thread Info.]
Archived from groups: rec>photo>equipment>large-format (more info?)
On 2/22/2005 6:03 AM Nicholas O. Lindan spake thus:

 > "Richard Knoppow" <dickburk.TakeThisOut@ix.netcom.com> wrote
 >
  >> The cross screen method was in use for very many years
  >> but began to be replaced by "screened" film around the early
  >> 1940's.
 >
 > The 'screened film' (now just called a 'screen') was made by
 > making a continuous tone shadow-image of the ruled
 > cross-screen. The screen then has 'grey dots' that merge
 > into one another.
 >
 > In (not-so) modern lithography the negative is contact
 > printed with the screen on to lith film.

Small correction: halftone negatives were made directly from the
continuous-tone originals by exposing the (negative) film with the screen in
contact on top of it in the process camera (in other words, through the
screen). The negative was then used to expose the (positive) printing plate in
the usual manner.

As you alluded to, almost nobody does this anymore.

(It would be possible to make a screened positive as you describe above, but
that would then require another internegative in order to produce a positive
plate. I don't know if anyone ever did it this way.)


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jdbeyer

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Since: Jun 23, 2004
Posts: 258



(Msg. 17) Posted: Tue Feb 22, 2005 3:11 pm
Post subject: Re: Square pinhole [Login to view extended thread Info.]
Archived from groups: per prev. post (more info?)
David Nebenzahl wrote:

 > (It would be possible to make a screened positive as you describe above,
 > but that would then require another internegative in order to produce a
 > positive plate. I don't know if anyone ever did it this way.)
 >
What I do to make screened halftones is to take the continuous tone
negative made of the subject and put in in my 45MCRX enlarger with a 180mm
Componon-S lens in it. I normally do this with 4x5 negatives.

In my enlarging easel, I put an 8x10" sheet of Kodalith Type III 2556
film. Above the Kodalith film I put a half-tone screen. The height above
the Kodalith depends on the contrast ratio of the continuous tone
negative. Most people who do the process always use the same height, and
this reveals their ignorance of how the process works. To control the
contrast, they pre-expose, flash, and do all manner of foolishness to make
the best of a bad job. Then they expose the printing plates and turn the
resulting mess over to a highly paid employee, called a fine etcher, to
try to repair their mistakes.

If you really know how to run the process, all you need do is set the
screen to the correct height and you do not need to pre-expose, flash,
fine-etch... .

The result I get is a positive screened image whose only problem is that
it is composed of "soft dots" and if you make a printing plate from that,
you will get all the wrong tones because it is too difficult to control
the growth of dot size (or shrinkage). So I contact that with another
sheet of Kodalith to get "hard dots" but now I have a negative. Depending
on the type of printing plate being used, you need either a negative or a
positive. If I need a positive, I can contact print a second time with no
problems controlling the dot sizes.

--
.~. Jean-David Beyer Registered Linux User 85642.
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jdbeyer

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Since: Jun 23, 2004
Posts: 258



(Msg. 18) Posted: Tue Feb 22, 2005 3:16 pm
Post subject: Re: Square pinhole [Login to view extended thread Info.]
Archived from groups: per prev. post (more info?)
Richard Knoppow wrote:
 > "Richard Knoppow" <dickburk.RemoveThis@ix.netcom.com> wrote in message
 > news:1108985232.562617.268720@f14g2000cwb.googlegroups.com...
 >
  >>Lobster wrote:
  >>
   >>>Just curious... what would be the effect of a square
   >>>pinhole (or a
  >>
  >>pinhole
  >>
   >>>the same aspect ratio as the film format). Would
   >>>vignetting still be
  >>
  >>radial?
  >>
   >>>Would overall sharpness be less? Would overall sharpness
   >>>be
  >>
  >>less/more/same
  >>
   >>>than an equivalent circular pinhole of the rectangular
   >>>diagonal?
  >>
  >>Could one
  >>
   >>>get a zoneplate made from concentric rectangles to do
   >>>anything
  >>
  >>useful?
  >>
   >>>Any idea what the effects might be anyone.
   >>>
   >>>JH
  >>
  >>
  >> The shape will make little difference to the image
  >>although it will
  >>affect the shape of out of focus highlights. Actually
  >>square stops have
  >>been used in the past. One of the simplest variable
  >>diaphragms is made
  >>by sliding two wedges against each other. This forms a
  >>square of
  >>varying size. This sort of stop was used in some very old
  >>cameras.
  >> There is some theoretical difference in the effects of
  >>diffraction
  >>but in practice they are not significant in normal
  >>photography.
  >> In another reply to this is mentioned the special
  >>stops used in
  >>making cross-screen half tone negatives. This is the
  >>method used for
  >>years for printing photographs by letterpress. The stop of
  >>the lens is
  >>actually imaged on the film by the cross hatch of the
  >>cross screen,
  >>which is located near the film. It acts like a
  >>multiplicity of pin-hole
  >>lenses. It was discovered that shapes other than round
  >>were superior
  >>for this purpose. Most process lenses have a slot for
  >>inserting special
  >>stops. However this effect does not apply to normal
  >>photography.
  >>
  >>Richard Knoppow
  >>Los Angeles, CA, USA
  >>dickburk@ix.netcom.com
  >>
 >
 > I realized after posting this that the question was about
 > pin holes, not lens stops. However, my answer is still
 > applicable. In fact the shape of the pinhole probably does
 > affect the diffusion pattern somewhat. If one looks on books
 > on optics one will find descriptions of the effect of
 > diffusion from round holes, slits, arrays of slits, and, in
 > some books other shapes.
 > The shape of the image of an object will still be the
 > shape of the object but the surrounding diffuse field will
 > change shape.
 >
 >
Diffraction does matter in fine screens, but probably not with a newspaper
(65 lines/inch) screen.

It can be instructive to make a crossline screen with a single sheet of
glass and some drafting tape. I made a 5 line per inch screen using 1/10"
wide tape. It actually works and the dots are big enough to examine with
the naked eye. You can learn a lot doing that, and it is way way cheaper
than a ruled glass sealed crossline screen.

If you want to make useable one, though, you might get two Ronchi rulings
at the right pitch and cement them together with a suitable optical cement
at right angles. I did that once, but I much prefer the two Max Levy
screens that the company gave me some years ago. I imagine they were worth
several thousand dollars a piece, but I do not know. They do not make them
anymore and donated their ruling engines to the Smithsonian.

--
.~. Jean-David Beyer Registered Linux User 85642.
/V\ PGP-Key: 9A2FC99A Registered Machine 241939.
/( )\ Shrewsbury, New Jersey <a style='text-decoration: underline;' href="http://counter.li.org" target="_blank">http://counter.li.org</a>
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lobster685

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Since: Oct 24, 2004
Posts: 9



(Msg. 19) Posted: Tue Feb 22, 2005 3:42 pm
Post subject: Re: Square pinhole [Login to view extended thread Info.]
Archived from groups: per prev. post (more info?)
"Richard Knoppow" <dickburk DeleteThis @ix.netcom.com> wrote in message
news:37vollF5hsughU1@individual.net...
 >
 > "Richard Knoppow" <dickburk DeleteThis @ix.netcom.com> wrote in message
 > news:1108985232.562617.268720@f14g2000cwb.googlegroups.com...
  >>
  >> Lobster wrote:
   >>> Just curious... what would be the effect of a square pinhole (or a
  >> pinhole
   >>> the same aspect ratio as the film format). Would vignetting still be
  >> radial?
   >>> Would overall sharpness be less? Would overall sharpness be
  >> less/more/same
   >>> than an equivalent circular pinhole of the rectangular diagonal?
  >> Could one
   >>> get a zoneplate made from concentric rectangles to do anything
  >> useful?
   >>> Any idea what the effects might be anyone.
   >>>
   >>> JH
  >>
  >>
  >> The shape will make little difference to the image although it will
  >> affect the shape of out of focus highlights. Actually square stops have
  >> been used in the past. One of the simplest variable diaphragms is made
  >> by sliding two wedges against each other. This forms a square of
  >> varying size. This sort of stop was used in some very old cameras.
  >> There is some theoretical difference in the effects of diffraction
  >> but in practice they are not significant in normal photography.
  >> In another reply to this is mentioned the special stops used in
  >> making cross-screen half tone negatives. This is the method used for
  >> years for printing photographs by letterpress. The stop of the lens is
  >> actually imaged on the film by the cross hatch of the cross screen,
  >> which is located near the film. It acts like a multiplicity of pin-hole
  >> lenses. It was discovered that shapes other than round were superior
  >> for this purpose. Most process lenses have a slot for inserting special
  >> stops. However this effect does not apply to normal photography.
  >>
  >> Richard Knoppow
  >> Los Angeles, CA, USA
  >> dickburk DeleteThis @ix.netcom.com
  >>
 > I realized after posting this that the question was about pin holes, not
 > lens stops. However, my answer is still applicable. In fact the shape of
 > the pinhole probably does affect the diffusion pattern somewhat. If one
 > looks on books on optics one will find descriptions of the effect of
 > diffusion from round holes, slits, arrays of slits, and, in some books
 > other shapes.
 > The shape of the image of an object will still be the shape of the
 > object but the surrounding diffuse field will change shape.
 >
 >
 > --
 > ---
 > Richard Knoppow
 > Los Angeles, CA, USA
 > dickburk DeleteThis @ix.netcom.com

Good example here of bright point sources appearing to image the zone plate,
though curiously the outermost diffraction ring is larger.
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