[pure-silver] Re: Densitometer for print exposure calibration

Phil Davis's "Beyond the Zone system" is the definitive work on this
subject.
http://www.btzs.org/ is the web site for articles and info on the book, and
software

http://www.btzs.org/Software/Plotter.htm This windows base software plots
both film and paper curves and the matcher feature graphically compares the
film and paper relationship.

I have seen some really good buys on eBay for densitometers that have the
ability to measure both film, and paper densities. 


http://www.btzs.org/Software/ExpoDevPalm.htm 
I have used a Palm Pilot loaded with info from Plotter software to determine
exposure and film development times. The Palm creates a log that saves some
time with note taking.

I will admit it took some effort to get up to speed on all this. This system
has provided me with a method to use Subject Brightness Ratios and metered
EV values to determine film exposure and development that yield negative
density ranges to match the papers I use. 
  
 
 "The Book of Pyro" by Gordon Hutchings 
 "Edge of Darkness" by Barry Thornton 
Both of these authors write about there experience with staining developers
and VC papers.
Barry Thornton wrote about the effects of stain color and there effects on
VC papers. He also formulated different developers that optimized the stain
for VC papers
Now available from,
http://www.photoformulary.com/DesktopDefault.aspx?tabindex=0&tabid=1

The BTZS Plotter software will graph projected or contact prints from step
tables. The density values from the stained negative can bet entered in the
step table column and the corresponding reflective print densities enter to
complete the plotting.
Unfortunately the negative density values are not linear like a step table;
therefore I suspect this would skew the curves.
I have used transmission densitometers, reflection densitometers, and on
easel photometers to determine negative density range, paper log-exposure
range, and paper exposure,  only to find the local gradation causes me to
change exposure, and paper grades (paper log-exposure range). To make an
analogy all this testing gets me in the ball park for sure, I see fewer
balls, and strikes, and some times I get a home run. 
   Remember negative exposure and development are finalized upon completion
of development, and all this effort is to make fine negatives that are a
delight to print. 

Jonathan Ayers [mail1&redwoodhorses.com]





-----Original Message-----
From: pure-silver-bounce@xxxxxxxxxxxxx
[mailto:pure-silver-bounce@xxxxxxxxxxxxx] On Behalf Of Richard Knoppow
Sent: Saturday, November 10, 2007 12:16 PM
To: pure-silver@xxxxxxxxxxxxx
Subject: [pure-silver] Re: Densitometer for print exposure calibration


----- Original Message ----- 
From: "vellum" <vellum@xxxxxxxxxxxx>
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Saturday, November 10, 2007 6:02 AM
Subject: [pure-silver] Densitometer for print exposure 
calibration


> Shannon's interesting thread about multigrade paper has 
> made me want to
> test my enlarger and paper to better understand it and 
> calibrate my
> system.  It's something I've never done before.  I'm fuzzy 
> on some of
> the concepts and what they really mean, like Log Exposure 
> Range, but
> I'll figure it out as I go.
>
> I have a Jobo Colorstar 3000, but I don't have the B&W 
> density probe for
> it.  I just have the standard color probe.
>
> What densitometers might be the best candidates for B&W 
> print exposure
> densitometry if I were shopping used on ebay?  I'd rather 
> buy a new one
> but I suspect new ones may be costly.  Ease of use on the 
> enlarging
> easel and accuracy (or consistency) would be priority.  An 
> exposure
> timer function would be nice, but not essential.
>
> Ability to use it to measure density of negatives would be 
> nice too.  I
> develop using both MQ and pyro developers.  Having one 
> densitometer that
> "does it all" would be cool, but probably there may be 
> none that do
> everything well.  In that case, it is the printing 
> densitometry that
> matters most to me.
>
> Skip.
>

     I think the key here is understanding that there are 
two media to be measured: negative and print. A transmission 
densitometer will allow you to measure the densities, but 
more important, the contrast, of the negative (with some 
qualifications). The resulting range of densities on the 
paper will also depend on the characteristics of the 
enlarger. Nearly all transmission densitometers measure 
diffuse visual density. Some enlargers use semi-specular 
(condenser) light sources which produce different effective 
densities than a diffuse source. This is called Callier 
effect and varies with the film. It is nearly absent for 
color film and greatest for coarse grain silver film.
    To measure the print one needs a reflection 
densitometer. A number of these have been made and are 
available. Both types of densitometers are made for either 
white light or standardized colors.
    The effective density of a negative with a stain image 
such as produced by Pyro is dependent on the color 
sensitivity of the printing material. If its printed to a 
panchromatic material the effect is nearly absent. For blue 
sensitive material the effect is to increase the contrast 
but not the shape of the film curve. For variable contrast 
materials the stain image may look like a masked filter 
causing the contrast to vary with the negative density. 
Since the filtering effect is strongest in the dense regions 
the effect is greatest on the print highlights. Since VC 
paper prints lower contrst for blue exposure than for green 
the effect will be to lower highlight contrast in about the 
same way a shoulder in the film characteristic would do. The 
amount of this effect willd depend on the relative density 
of the stain versus the silver density, the spectral 
characteristics of the paper, and the light source.
    Beacause the paper characteristics affect the resulting 
curve it is difficult to measure using a densitometer. You 
would have to have filters at the correct colors to match 
the two components of the printing paper.  I don't think the 
blue and green filters used for color work are at the right 
wavelength.
    One could presumably print a step tablet onto the film, 
process it, and make prints from which a reflection 
densitometer could measure the resulting density curve. I am 
pretty sure I have seen published results of such tests but 
can't cite a source.

---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx 

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