Beakman (beakman@netcom.com)
Sat, 22 May 1999 20:46:22 -0700 (PDT)
> <<and I can tell you that imagesetters are calibrated for PERCENTAGE
> DOT but NOT for density.
>
> Hmmm.... the percent dot has a direct relation with density; if we are
> talking about hard dot, theoretically as long as the Dmax is above 2.0 then
> the difference should not make any difference. That is, 50% dot should
> measure density of 0.30 (+base+fog) if the Dmax is above 2.0.
>
> Of course, if the Dmax is really as low as 2.0, it could mean that the dots
> are not that hard because each dot actually has a gradation from Dmax to
> Dmin, but imagesetter's Dmax usually is above 3.0.
>
> >> The density ranges all over the place, and as
> the imagesetter runs, and the lasers heat up, it changes.
>
> I find this interesting and curious. It sounds like the dots are not hard.
>
> >> The change in
> density will not effect the percentage dot values (as used in offset
> printing) hence it is of no concern to printers. HOWEVER, the density IS
> important for alt-photo work because the negs print as a hybrid. >>
The Dmax of the film typically ran from 4.0 to 7.0 (!) -- incredible, but
true. Suppose you ran film and made a box filled with 1% dot on the neg.
and next to it a box with 0% dot (i.e. opaque film). Then let's say that
you measured the optical density of the opaque square and it measured 4.0.
Then you measured the % dot of the 1% box, and sure enough it is 1% dot.
Next you measure the OPTICAL density of the 1% box and find that the
reading is say, 1.8. O.K., now you crank up the lasers and run another
neg. This time the optical density of the opaque square is 7.0, and the
OPTICAL density of the 1% box is 3.1 EVEN THOUGH THE % DOT STILL MEASURES
1%! As it turned out, for me anyway, the OPTICAL density was important.
> Do you use stochastic screening or traditional halftoning?
I use 45-degree elliptical dot halftoning. If you like, you can read all
about it in a paper I wrote which is posted on the Bostick & Sullivan web
page. The only change I would make to the paper would be to suggest
sacrificing some shades of gray in order to use a finer linescreen -- try
400 or 425 lpi -- you may not notice the loss in shades of gray, but the
prints may be better.
best regards,
David Fokos
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