Re: halftones

[P3D RJ Thorpe <thorpe@xxxxxxxx>]

Marvin Jones writes
> > Peter Davis writes
> > >practice of halftoning was originally done by projecting a photograph
> > >through a piece of glass with a finely etched grid of lines.  The
> > >transmission properties of that grid resulted in the halftoning effect.
> > >The piece of glass had no intelligence or algorithm to accomplish it.
> >
> > Gregory J. Wageman writes
> > >That's not halftoning.  I'm not sure what it is (besides possibly
> > >just an 'effect') but it isn't halftoning.
> >
> When I worked for a time in a print shop many years ago, that's exactly how
> we created halftones, except we used a sheet of plastic gel with a fine dot
> pattern of a certain density, depending on the dpi we wished to achieve in
> the final product. My boss at the time had been producing them that way
> since the '20s, and I know they are still produced that way now. In fact,
> I've never heard of any other method of producing them!

I missed the origianal thread so I don't know exactly what Mr Davis was
refering
to. Perhaps this was one of the original methods of halftoning that has
since
evolved.

Mr Jones's explanation is pretty good in its general thrust but it is
wrong in
just about every detail. His explanation is a mixture of printing tints
and
halftones. Tints have a "certain density", black. The size of the dot
yields a
certain percentage tint for any particular number of "lines (or dots)
per inch".
Halftones screens, on the other hand, have a similar "dot size" and
"lines 
per inch" structure, but they fade in and out across the gray scale. The 
centers of the dots are black (dense) and the centers of the space
between the dots is clear (not dense). The transition is gradual. The
brighter the light, 
the more it "burns" through the screen. Ortho film can only produce
black and 
white, no grays. So the brighter an area is in the original photo, the
greater 
effect it will have on the dots in the halftone neg.

Many prints shops still use this method when shooting through a copy
camera.
But I haven't specified this process for years. Nowadays, Just about all
prints
are scanned and inserted electronically into page makeup programs like
PageMaker
or Quark. The service bureau produces the halftones or color separations
simply
by running the film out the same way as if there were no pictures in it
at all.
The computer program takes care of it all.

-- 
RJ Thorpe
Cedar Rapids, IA
mailto:thorpe@xxxxxxxx
http://www.skep.com


------------------------------