U of S | Mailing List Archive | alt-photo-process-l | Re: Gum calibration (was: Paper negatives- Ink Selection)

Re: Gum calibration (was: Paper negatives- Ink Selection)



Hi Loris,

I think I am beginning to understand where you are going with this thread.

Thanks for bringing density range into the discussion, that was very helpful.

My earlier comment regarding Standard Exposure Time being dependent upon the base density of the substrate for the negatives and not on printer or inks was in response to a comment K Thayer made.

You are correct, the curve has nothing to do with the calibration step you are talking about.

However, the density range of the negative does not affect the exposure.  Exposure is still merely dependent upon the UV density of the substrate you are printing on.  Once the exposure is determined, then you just match the density range of the negative to the exposure scale of the contrast mix you are using.  If, for some reason you have a fixed density range in the negative, say from using all inks to make a "grayscale negative", then you must alter the contrast mix so that its exposure scale matches the density range of the negative.  If you end up with a density range that is too high, your highlights will be blown out.  However, if you try to bring tone into the highlights by increasing exposure, you end up over exposing the shadows and blocking them up.  What should be done in this case is to either increase the exposure scale of the contrast mix (by adding more chromate) to match the negative or reduce the density range of the negative, as you have suggested, to match the exposure scale of the contrast mix.

You are quite right in saying that if you and Keith are using different printers, inks, and drivers, you will get different Density Ranges in your negatives.  Even the same printer and inkset and driver settings on two different printers will give a slight variation.

Best Wishes,

Mark Nelson


On Oct 18, 2008, at 1:50:41 AM, "Loris Medici" <mail@loris.medici.name> wrote:
From:"Loris Medici" <mail@loris.medici.name>
Subject:Re: Gum calibration (was: Paper negatives- Ink Selection)
Date:October 18, 2008 1:50:41 AM CDT
To:alt-photo-process-l@usask.ca
It's not about curves Katharine, why you insist that much to NOT
understand what I'm saying? I'm talking about PRINTER / PAPER PROFILE,
INKS, NEGATIVE COLOR and most importantly, the specific DENSITY RANGE
which one can get from that combination here, since from the start. No
twisting please. Whew!

Besides, we have a comprehension problem here: Mark says that in condition
of "...providing all other variables are equal...", now, we're comparing
exposure time where dichromate concentration is similar, lightsource is
similar, negative substrate, paper, gum and stock gum solution strengths
are the same, what is considerably different between Keith and I (which
you are ignoring) here is the printer (therefore paper profile & inks) and
I'm completely sure that Keith's negatives DR are greater than mines and
if he calibrates for a lower DR, he'll get shorter exposure times...
Designing a curve will be just a side bonus.

OK, I decided to take another road, since the people doesn't get it with
the current one: Keith, can you please send me a small image (say 4x4" @
300dpi, 16bit grayscale) privately, if you like? I will make a negative
for you from that one and send it to you allowing you to test it (along
with the negative you personally made from that file) and see what
happens? (Or, you can just have it measured with an UV densitometer - that
also will give an idea...)

Regards,
Loris.


18 Ekim 2008, Cumartesi, 3:25 am tarihinde, Katharine Thayer yazmış:
>
> Keith, I've said this before, but since it keeps coming up I'll say
> it one more time: if your goal is to reduce exposure times, this
> isn't the most fruitful place to look IMO.
>
> Mark has pointed out that exposure time isn't a function of printer,
> ink type, ink color or printer settings:
>
> ender100 wrote:
> > Exposure time does not depend on the printer, ink or color of the
> > negativeâ&#65533;&#65533;it merely depends on the substrate you are
printing
> > onâ&#65533;&#65533;providing all other variables are equal.
>
> and logic says that exposure time can't be a function of curves,
> since the exposure time is established before the curve is
> calibrated, and doesn't change after the curve is calibrated.
> Exposure time is a function of a lot of things such as light source,
> dichromate concentration, pigment concentration, paper speed,
> environmental conditions; how the negative is generated is not a
> significant factor, in my experience.
>
> Katharine




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