U of S | Mailing List Archive | alt-photo-process-l | Re: digital negative possibilities for gum

Re: digital negative possibilities for gum

On Oct 17, 2006, at 4:18 PM, zphoto@montana.net wrote:

OHHHH totally good point:  I was using Image>Mode>bitmap.
Now I have another thing to try--the diamond thingy, below.
So many things to test...but thanks for the input
You're welcome, but I'm not sure you understood my point. There's one more branch beyond Image>Mode>Bitmap: you still need to specify >type of bitmap ( diffusion dither, pattern dither, custom dither, halftone screen, or whatever). When I open the dialog box, "diffusion dither" is the option that's selected on that menu so I'm guessing, though I don't know for sure, that diffusion dither is the default, and since you didn't mention having chosen another option, I'm also guessing that diffusion dither is probably the kind of bitmap you made, by default. Diffusion dither is a stochastic dot, the same kind of pattern you get from your Epson printer. So if that's what you used, you're just comparing one kind of stochastic dot printed with a different kind of stochastic dot, to the usual stochastic , which is not the same as, say, comparing a halftone screen (with a round dot and the screen angles set to deposit the different gum colors in different sections of the dot) to a stochastic dot.

Today I printed a bitmap negative from an image I printed in gum yesterday (from a normal inkjet negative) choosing Image>mode>bitmap>360>diffusion dither, from a file that was 360 ppi. Both negatives were printed on cheapie inkjet film (something nameless I bought online for something like $16 for a box of 50). I've uploaded scans of both of these negatives, with prints made from them.

What amazes me is that you got such a good print from uncurved bitmaps, because in my experience printing gum from dififusion dither bitmaps, this kind of file requires an extreme curve, otherwise the negative will be way too dense and print with too much contrast. As you can see from the example, the uncurved bitmap was so dense in the face area that most of the details of the face washed away almost immediately (the two negatives were given the same exposure time, as per your protocol) just as I would have expected. So I'm fascinated that you were able to print uncurved bitmaps with apparently no problem, and maybe my guess that they were diffusion dither bitmaps is mistaken. (But if not, then what?) At any rate, I would strongly suggest to anyone considering diffusion dither bitmaps (although I'm sure I don't know why anyone would) that you use a curve. The curve on Dan's CD (first edition) for preparing a bitmap for the imagesetter for silver printing was the one I used, with some modification, when I was printing bitmaps for the laser printer for gum printing.

Marek, you can see how grainy the bitmap negative is. It's not quite as easy to see the dots on the print in the jpeg, although they can be seen by eye. And through the 8x loupe they are very pronounced. People who have come to alt-photo fairly recently might stop and consider that earlier in my days of gum printing, this was the best one could do as far as making negatives with a desktop printer (imagesetters, which Dan and his followers used so effectively in those days, weren't available within 100 miles of here, so I made do with what I had at hand). I don't see any point to making this kind of negative now when you can do so much better with today's inkjet printers using a normal file, even on cheap transparencies.

Yes, I had to use bitmaps when I was printing negatives on a laser printer, but it was the ONLY way to get a stochastic dot on a laser printer. Now we've got printers that produce stochastic dots, and much smaller and finer ones at that, all by themselves. As your comparison shows, even on a cheap transparency the unbitmapped negative prints gum at least as well as the bitmapped, and in my example there's no comparison; the unbitmapped is far superior to the bitmapped. And even if it were properly curved, there would still be the grittiness in the print. So I'm scratching my head to try to understand what possible advantage could accrue from using bitmaps now.

I looked through an 8x loupe at the two kinds of negatives; they look quite similar, except that the bitmap one looks like a magnification and intensification of the other one. The dots are similar in shape, except the ones on the bitmap seem a little more square (pixel- shaped) and the regular ones seem maybe a little more rounded, but I'd need a 10X loupe, as Marek says, to be sure about that, since they're so much smaller. But as I said, the bitmap looks like a magnification of the regular negative. The dots are bigger and darker, as I would expect in an uncurved bitmap, but the shape and distribution of the dots seems quite similar. (BTW, I'm curious what's the basis for saying that bitmap uses less ink? I would say it uses way more, especially an uncurved bitmap.)

Oh, the URL. This is a big jpeg, so will take a bit to load, and for that reason also it won't stay on the site long, as bitmaps don't seem terribly relevant to modern-day printing, to me anyway, so it's not something I'm willing to give that much space to longterm.



Marek, yes, you can actually see the minute dots by the eye
with gum!  So those who say gum is not sharp--heck--it sure
is.  But what I mean is that the little dots, the softness
of the image under bitmap is actually kindof...pleasing.  I
dunno--it's weird, but I was very impressed at the feel of
the image with this kind of neg.  I'll try the other stuff
this week and report back.
----- Original Message Follows -----
From: Katharine Thayer <kthayer@pacifier.com>
To: alt-photo-process-l@usask.ca
Subject: Re: digital negative possibilities for gum
Date: Tue, 17 Oct 2006 08:39:59 -0700

On Oct 16, 2006, at 8:07 PM, Katharine Thayer wrote:

Chris, what kind of bitmaps were you using, a halftone
screen,   diffusion dither, or some other?

The reason I'm asking is that I'm not sure what it is
you're   comparing when you're comparing a negative made by
printing the file   as a bitmap file to a negative made by
printing it as a regular file.

The assertion I've heard (Mark is the last person I
remember making   this assertion here) has to do with
halftone separations  rather than   bitmaps per se, and
goes something like this:  gum printers "have   found" that
halftone separations give better clearer colors because
the color is laid down next to each other rather than on
top of each   other.  I said that might be true of opaque
pigments, but certainly   not of transparent pigments,
which can be printed directly on top of   each other
without muddying the color.

With halftone separations, the screen angle for each of the
 separations is set so the color is laid down in a rosette
pattern,   each full dot being made of a rosette containing
each of the three   colors, like three different-colored
petals making up a flower. In   that case, the color really
is laid down next to each other rather   than on top of
each other, and would give you the comparison you seem   to
be after.  But if you were using halftone separations of
this   kind, it seems like you would have said so, rather
than   characterizing the negative type as a "bitmap."

So, some clarification would help me understand what it is
I'm   looking at here, thanks.


On Oct 16, 2006, at 6:57 PM, Christina Z. Anderson wrote:

Good evening all,

Over the last week I have been testing four kinds of gum
negs,   with tricolor
gum and gum over cyanotype (cyanogum). My goal was to
see if   bitmapped negs produced clearer, more brilliant
colors as I read   somewhere, or even worked with gum,
and then to find an   acceptable, cheap, low tech
beginner mode of gum printing.
I made sure to actually attach the negs side by side so
all other   variables
were exactly alike--coating, dry time, development, etc.

So here's the skinny:
1.  Trigum printed with a negative on cheapy Photo
Warehouse OHP transparency with all inks, no curves
2.  Trigum printed with a negative on cheapy Photo
Warehouse OHP transparency with all inks, no curves, and
bitmapped 360 ppi input   and
3.  Cyanogum printed with a negative printed on
expensive   Pictorico, all
inks, no curves
4.  Cyanogum printed with a negative printed on
expensive   Pictorico, curved
correctly for cyanotype, magenta and yellow separately,
colorized neg
These are my observations (NOT declarations or
assertions); YRMV: 1.  Bitmapping surprised me--it
actually produced a pretty darn   good image!
It was softer, a bit less contrasty, but heck, with what
little ink bitmapping uses and with the fact you can use
cheapy transparency,   it is
definitely a keeper,especially for teaching beginners
low tech gum. 2.  "All inks" was a bit smudgy and
required drying with the PWOHP/  Epson
2400, not with Pictorico.  Funny, my cyano layer printed
with   minute round
spots of lighter tone--not the dreaded speckles--and
when I louped   the
negative I saw that the printer lays down minute round
spots of   different
color inks that in turn expose cyano differently, like
little mini   filters.
Very interesting. With gum this is no problem--the
spottiness, of   course.
3.  Cyanotype absolutely requires a curve--by the time
the   highlights are
printed in, the shadows are totally overexposed unless
your image   is short
scale to fit the 4 or 5 stops of that process's range.
My next   test is to
curve just the cyano and use the two bitmap magenta and
yellow   negs to print
gum over. And then next I will probably curve the
individual negs   and then bitmap.
4.  If not printing with a cyano underlayer, you can get
an    acceptable
print with no curves, neg just inverted and printed as
is, and   adjust the
layer with exposure, development, pigment load,
brushing. But all   of you
already knew that, I'm sure I'll be told. I prefer the
all inks   unbitmapped
to the bitmapped--I think.
5. In my eye the better print was produced by a properly
curved neg, but how will your viewer know there is a
"better" rendition unless   all the
images are side by side, you know?
6.  Bitmapping didn't produce clearer, better colors
because of   "individual
dots laid down side by side and not on top of one
another". 7.  All methods can be capable of producing
fine prints, once the   gum printer can meld his/her
method to whatever workflow is chosen. 8.  Bitmapping
has....possibilities...I'm not sure what yet, but   it
really did surprise me.
If you want to see the visual, copy and paste this URL
into your   browser:


and scroll down to the very bottom of the images; it'll
be there. Bye!

Assistant Professor of Photography
Photography Option Coordinator
Montana State University
College of Arts and Architecture
Department of Media and Theatre Arts, Room 220
P.O. Box 173350
Bozeman, MT 59717-3350
Tel (406) 994 6219