|
|
|
|
|
|
|
|
Re: Pyro Developers Question.From: dickburk@ix.netcom.com (Richard Knoppow) "Lew" <lew1716@gmail.com> wrote in message news:ec481232-75a4-4eee-93ec-bbec5eeee1de@u26g2000yqu.googlegroups.com... What about film speed? Are there any fine grain, staining I don't know. Imagewise staining is produced by some phenol type developers as is imagewise tanning. Pyrogallic acid and Pyrochatichin (there are other spellings) are formost among the staining developers but hydroquinone can cause imagewise tanning without the staining. Pyro has been mixed with other developing agents, particularly Metol, but that was done beause the two tend to preserve each other and extend the life of the developer. Curiously much of the reseach done on Pyro developers before they were pretty much supplanted by M-H types was to devise non-staining developers. The reason is that the stain image affects the printing density in a way that varies with the spectrum of the printing illumination and with the spectral sensitivity of the printing material, plus it can be difficult to measure because the densitometer used must match the spectral sensitivity of the printing material. All is much easier when the image is neutral. You can get some idea of the increase in effective density for blue-sensitive materials by looking at the negative though a blue filter. Its also possible to bleach out the silver image which leaves only the stain image. That is one way of determining the effectiveness of a developer in forming a stain image. The effect desired by most workers using current materials is the self-masking effect of the stain image when used with variable contrast materials. It tends to lower the contrast of the highlights flattening out the very high contrast produced by some modern films. In effect it creates a shoulder on the film characteristic. Might or might not be desirable and may not be much of an effect on some material. Grain is affected by many variables: the activity of the developer is one, generally less active developers are less grainy; another factor is the pH of the developer, higher pH (more alkaline) tends to produce coarser grain; the presense of halide solvents affects grain somewhat but not for the reasons popularly supposed. The solvent, sulfite in most developrs, does not "etch away the grains" but rather affects the way the crystals grow as they are developed. Up to a certain point the presense of a solvent will increase film speed because it makes more development centers available to the developer. This is one reason that D-76 has become the reference standard for film speed. When more solvent is present, or a stronger solvent such as sodium thiocyanate, is used the film speed goes back down because the solvent can destroy some of the latent image centers. The total range of speed variation from "normal" type developers is about 3/4 stop more to 3/4 stop less than D-76. Some Phenidone developers, notably Xtol, T-Max RS, Microphen, as examples, increase speed about 3/4 stop. Some very fine grain developers such as Perceptol and Microdol-X decrease it about 3/4 stop. About the only developers which loose more speed than this are the old fashioned paraphenylenediamine super fine gain developers. A pure PPD developer (with no other agent added) can loose as much as 5 stops! These developers can also produce severe dichroic fog on modern films and do not, for the most part, produce any finer grain than Perceptol or Microdol-X In general, film speed and fine grain are not compatible. Probably the best compromise is Xtol which yields slightly finer grain and slightly higher speed than D-76 along with good sharpness and good tonal rendition. Whatever reliability problems it had in the past seem to have been cured. -- Richard Knoppow Los Angeles, CA, USA dickburk@ix.netcom.com
|
