Yves,
The gum variables can be modified to change the Exposure Scale of the
printing. If you first adjust the density range of the negative prior to applying a
curve, then you will get the maximum from the gum print—then the curve is
only used to adjust the relative tonalities between DMax and Paper White.
The adjustment of the density range of the negative is pretty important for
gum, since it has a shorter exposure scale than many other processes and
requires a lower density range negative. Doing this is one (just one) reason why
Chris Anderson is having so much success with her tri color gum thingies.
An example might be making a negative with an Epson 2200 where the UV
transmission density of a negative using all inks can be over log 4.0—so if you need
a negative to match an exposure scale of gum at let's say log 1.2, then you
have a mismatch of 4.0 - 1.2 or log 2.8 TOO much density in the negative that
the curve has to adjust for—that is over 7 stops! This is why you often see
people using curves where the endpoint has been moved to reduce the density so
the highlights won't blow out... unfotrunately for every point that you move
that endpoint, you lose that many tones in the negative. You can actually
use this to calculate exactly how many tones will be lost.
Best Wishes,
Mark Nelson
Precision Digital Negatives--The Book
PDNPrint Forum at Yahoo Groups
www.MarkINelsonPhoto.com
In a message dated 4/28/06 10:43:12 AM, gauvreau-yves@sympatico.ca writes:
> Katharine,
>
> my first reply on this topic was probably the cause of the misunderstanding,
> when I read it back now I see what you mean. With the last one I thought I
> made all this as clear as I can but I'll try again. If whatever you do back
> exposing your print fails to give you a satisfying tonal "delicacy" as you
> put it, may be applying a different curve would help.
>
> If I understand normaly exposed gum printing (front exposed) you can control
> the distribution of pigment (tonal "delicacy") by the various usual means
> including % gum, % pigment, % dichromate, thickness of emultion,
> exposure(s), development and physical manipulations, etc. With back
> exposure, it seems only one exposure can be done and all I'm saying is that
> beside all the usual controls you have the possibility to change the
> negative density (distribution) by applying some curve. Can you control
> every thing with some curve, the answer is simple no. The reason for this is
> that a couple variables of the gum process are totally independent of
> exposure (negative densities), the pigment load, as you call it, is one of
> these, development and physical manipulations are other mean by which you
> can alter the tonal distribution, in the limit you can scrape it all off
> (the emultion).
>
>
> I would certainly claim that if you maintain every variables fix ie. you
> don't change anything from print to print except the curve applied to the
> negative, you can basically obtain any tone you want between the Dmax and
> the Dmin of the print. Obviously, this fix variable gum print must show
> something usable to begin with.
>
> Regards
> Yves
>
Received on Fri Apr 28 11:49:15 2006
This archive was generated by hypermail 2.1.8 : 05/01/06-11:10:26 AM Z CST