[pure-silver] Re: Developer and Neg Density


----- Original Message ----- From: "Adrienne Moumin" <photowonder2010@xxxxxxxxxxx>
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Thursday, June 22, 2006 2:41 PM
Subject: [pure-silver] Developer and Neg Density


Mark Blackwell's question about neg density brought to mind something that's been on my mind since I set up my darkroom 1-1/2 yrs ago.

I have a Saunders LPL 4550XLG enlarger w/the color module, and it has been a stretch to get used to it since I've almost always printed on condensor enlargers. I consistently find that the light is alot less contrasty than I am used to, as well as somewhat dimmer.

I use Edwal FG-7 w/ 10% sodium sulfite for fine grain. I didn't realize different developers influenced neg density the way Richard mentioned. I'm wondering whether I should also try D-76 for the extra 3/4 stop.

Thanks in advance for any advice,

Adrienne Moumin

PS-Christina, welcome back, and I love your work too!

Standard developers will all deliver the same contrast if the development time is correct. There ARE special developers for very high (like lith) and very low (like POTA or Technidol) contrast but otherwise its a matter of time.
Kodak time/temperature charts are given for a contrast index suitable for contact printing or diffusion enlargers. That should work OK for your enlarger because most color heads are diffusion sources. In a condenser enlarger these negatives may take one grade softer paper to print with the same tone rendition. Not a bit deal with modern VC papers.
The difference in _effective_ contrast is due to something called the Callier Effect. Callier discovered that the grains of silver scattered some of the light striking them. In a condenser enlarger the light is _collimated_ that is, it reaches the film in parallel rays. Some of this light is scattered out of the optical path, that is, away from going to the lens, so the densities appear to be higher. A diffuse source has ligth coming from all directions, or, more exactly, from most of a hemisphere. Some of this light is also scattered away from going toward the lens but light from other angles is scattered _toward_ the lens, so, the densities appear to be less than with a condenser source. The amount of Callier effect, sometimes also called Q factor, varies with the granular structure and thickness of the silver layer. Generally, it is somewhat greater for coarse grain film than for fine grain film. Color film, where the image is composed of very small dye particles, which are also transparent, has almost no Callier effect, so color negatives or slides print with about the same contrast regardless of the type of light source.
In any case, the contrast of the negative can be exactly compensated for by changing the grade of printing paper. In his well known book _Controlls in Black and White Photography_ Dr. Richard Henry shows tests he made of printing with both types of sources. In one case the negative contrast was adjusted and prints made on the same grade paper, in the other test the same negative was used and paper grade changed. In both cases the two curves lie exactly on top of each other.
In its data sheets for film Kodak usually gives a correction factor for negatives for condenser printing. Ilford curves are evidently compromise values which will print within about one half paper grade on either type. Agfa seems to have used diffuse values.
While the ISO does not specify a contrast index or average contrst in the speed standard for B7
&W sill film, it does specify a range of density to be obtained from a range of exposure, which amounts to the same thing. The effective contrast of the standard is about right for contact printing and diffusion enlarging. Since the speed is valid for only this value changing contrast by varying development also affects effective film speed. A reduction in contrast results in a reduction of effective film speed, conversely, increasing development to increase contrast also increases effective film speed.
Note there that one can NOT "push" film without increasing contrast or "pull" it without decreasing contrast.
As a rule of thumb, the amount of increase or decrease of development time for a one paper grade difference in contrast varies with the type of film. For conventional type emulsions it is about +/- 25% to 33%, for Tabular grain films, its less, around 15% to 25%. This more rapid change in contrast with development for tabular grain films accounts for some of the troubles people have with T-Max contast. Of course, the variation will also be greater with temperature variations and variations in agitation.

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




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