1. Did some definitive resolution tests. I found that using a 3600 dpi
imagesetter producing a 225 lpi linescreen (thereby resolving 256 shades
of gray) from a 450 dpi image file, produced pretty reasonable results.
You really couldn't "see" any dots with the naked eye. That said, I also
had made a 300 lpi image (made on a 5080 dpi imagesetter) from a 600 dpi
image file. I think the results, to the naked eye are excellent, and are
slightly better than the 225 lpi screen. Looked at with a loupe, you can
tell the 300 lpi image is definitely better, but who looks at prints that
way?
Anyway, I've decided to go with 317 lpi -- the maximum I can do on the
5080 dpi imagesetter and still get 256 shades of gray. My image files
will be 635 dpi. I can't see any reason to go to a linescreen higher than
this. By the way, the prints are turning out nice and smooth (creamy?),
with lot's of contact-print detail.
2. Important news. It is extremely important that you have the service
bureau calibrate the intensity of the imagesetter's laser to give you the
density range you want in the final neg. This will alter the contrast of
your neg.
Here's what you do...
a. Take your sensitized paper without any contrast agents -- no chlorate,
hydrogen peroxide, or dichromate (in the developer) -- this is to ensure
the least amount of grain in your final print. Expose a Kodak step tablet
(or equivalent) and develop normally. From this you will be able to
determine the desired density range of your negative.
b. Go to your service bureau and tell him/her/it that you want to make a
negative which includes a step tablet. This tablet should include a 1%
step. If you are using any of the curves I posted before you will notice
that anyting that is 0% (pure white in the print) or 1% gets set to 1%.
This means that anything that is pure white, will be output at 1% after
the correction curve has been applied.
Once you've gotten this 1% step, tell your service bureau that the
measured *density* (not "dot") of the 1% step minus the density of the
film base+fog, should equal the desired density range found in step a.
above.
This should be the very first thing you do with a service bureau once you
are ready to make negatives. The desired density range will probably be
different for different papers. Once you've got this dialed in, *then*
you can begin to refine your correction curve.
3. Also tried a 1200 dpi bitmap again, just to check if the "grit" I was
getting before might be due to other factors. Result: Still a litle
grittier than the halftone, but encouraging enough for me to try a 2400
dpi bitmap to see what happens.
Looked at with a loupe, the bitmap resolves more detail than the 225 lpi
linescreen, and about equal to, or slightly better than the 300 lpi
linescreen. If the grit doesn't completely vanish with the 2400 dpi
bitmap, then it'll be linescreens for sure.
That's all for now...
David Fokos