FDanB@aol.com
Date: 09/20/02-12:36:16 AM Z
You said in your message...
>I made a ramp from zero to 100% in Photoshop, then posterized it into 21
>steps.
Hey Judy,
Congrats on biting off a piece of this.
About Photoshop percentages:
As you probably know, Photoshop's 0%-100% refers to dot density in print
output: a 0% dot indicates no ink on the substrate and a 100% dot would
be black (assuming you're using black ink; a 100% reading of cyan would
be pure cyan). Let's call these Dot Densities, since they refer to the
size of the dot of ink on...something.
Now about Photographic Densities:
Normally, when we talk about step tablet "densities" we are talking big
"D" Density (which, for the math nerds among us is log transmittance), a
very different thing from Dot Densities.
Another assumption with the Stouffer step tablet is that Silver is the
light-blocking medium. Silver, conveniently enough, is very effective at
blocking both the visible parts of the spectrum (great for silver gelatin
printing) and invisible parts (perfect for UV sensitive processes like
platinum).
Densities in your Stouffer are, if memory serves, 1/2 stop (or 0.15
Density Units) apart. If your Stouffer has 21 steps, then about 10 stops
are represented.
Herein lies the problem:
When you make digital negatives, you use all kinds of material to a)
block the exposing light or b) filter the exposing light. For instance,
if you make imagesetter negs, your negative blocks light with silver,
much like (though not exactly like) a traditional b/w negative or your
Stouffer step tablet.
With inkjet negatives, you are usually using a dye, sometimes a pigment
and sometimes a polymer. These all block/filter exposing light in
different ways.
For example: the Epson 1280 drove me crazy for months because it flooded
the Pictorico OHP (a very fine transparency film) with too much ink. I
finally went back to creating an orange safelight color ink to apply from
Photoshop (like I had to way back in the Paleolithic period on the Epson
Photo EX) and can now make splendid desktop platinum negatives with stock
Epson dyes. For silver printers, the Pictorico PGHG White Film can make
desktop negs that print with near camera-original results from the 1280.
Though some might complain at the expense, $1.75 for an 8x10 negative
(and one with which you can finesse your contrast) strikes me as a
terrific bargain.
The carbon-based pigments that Jon Cone (Piezography) uses are very
effective at blocking UV but because the pigments don't adhere to the
transparency film well, a protective overcoat spray is needed.
Finally, getting back to your question: "Why does it print fewer steps --
on each of 5 substrates I'm testing -- than the Stouffer 21-step I'm
printing next to it?" And your second question addressed the uneven print
densities: " wildly uneven -- sort of grand canyon leaps at top, then
squished at the bottom."
Both these have to do with FOUR variables: your printer, your inks, your
substrates, and, finally...the wild-ass curve you must apply in Photoshop
to compensate for the first three variables.
If you don't start with one of the printer/inks/substrate/curve
combinations that I or someone else has test-driven, you're on your own
for printing step tablets and tweaking curves. And yes, it can be time
consuming. What I'm saying is that the "vellum" you're using may work
just fine once you discover the perfect combination. In the mean time,
why don't you start with one of the Pictorico/Epson combos that perform
well for silver or platinum?
Another question might be: why the dickens am I writing about digital
negs at 1:30am?
Hope this helps.
Dan
www.danburkholder.com
"Imagination is more important than knowledge."
- Albert Einstein
"Knowledge in moderation ain't bad either."
- Dan Burkholder
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