I wish to know the process of the negatives, as they are exposed to light how the image is stored. What chemicals are involved and what reactions take place? And as well, what happens chemically with the developer and fix?

-- Lara Rafton (dweege@goplay.com), April 26, 1999

Answers

Look for The Theory of the Photographic Process published by Macmillon (New York). I don't know if it is still in print because the copy I have is fairly old. You may be able to find it in your local university library. It covers such things as the structure of silver halide grains and their thermodynamic properties, latent image formation, the physical chemistry behind reciprocity effects, developing agents and their reactions, the electrochemistry of developers, kinetics of development, the structure of the developed image, and much much more. It pays to have taken physical chemistry in college before reading this book, but the general reader will probably find it interesting also.

-- Fritz M. Brown (brownf@idhw.state.id.us), April 27, 1999.

I forgot two other references that I tend to advocate for. Steven Anchell's Darkroom Cookbook and Steven Anchell and Bill Troop's Film Deveolpment Cookbook. The Film Development Cookbook has some really nice explainations of the various componants of developers, fixers, etc and what roles they all play in the effects of a particular developer. They describe the differences between such things as fine grain developers versus, say, high acutance developers. Both books have a pharmacopia of formulas and are a good set of books to have on the shelf just for the recipes if for nothing else.

-- Fritz M. Brown (brownf@idhw.state.id.us), April 27, 1999.

Lara, You didn't say what your background is, but unless you are a chemist, you really don't want to get into the details of photochemistry. It is far from a fixed and final issue. Many PhDs are working on it every day at companies like Kodak, Agfa, Fuji, etc. I think what you are really asking is for a source that will explain the interactions of chemical and film/paper from a practical control viewpoint. The above references are good. I would also suggest "Controls in Black and White Photography" by Richard J Henry. I have the second edition dated 1988. There may be a later one. This is a thorough discussion of the variables and interactions, which avoids long chemical equations and such, but does give you an understanding of the processes involved.

-- Richard Newman (rnewman@snip.net), April 27, 1999.

Wasn't it so that developer just speeds up the process of imagebuilding on a negative from latent to manifest and that if you wait long enough the latent image will become manifest without developer also? So, in this reasoning developer is catalyzing the reaction of light in the emulsion?

-- Lot (lotw@wxs.nl), April 28, 1999.

Francis Langford's Basic and Advanced Photography books from Focal Press also have good discussions about photographic physics and chemistry. Sorry I dont have ISBN's to pass along.

-- tony brent (ajbrent@mich.com), April 28, 1999.

Tony might mean Michael Langford. ISBNs 0 240 51257 X and 0 240 51088 7. Excellent books.

The Focal Encyclopaedia also decribes the chemistry, in far more detail than I can cope with.

-- Alan Gibson (Alan.Gibson@technologist.com), April 29, 1999.

Lot wrote: "Wasn't it so that developer just speeds up the process of imagebuilding on a negative from latent to manifest and that if you wait long enough the latent image will become manifest without developer also? So, in this reasoning developer is catalyzing the reaction of light in the emulsion?"

A little chemistry here: In the unexposed silver grain the silver is a monovalent cation (Ag+). Light catalises the limited reduction of the some of the silver to Ag(0) (metalic silver). Given enough light intensity and time, the image will form to some degree solely from this reaction. As an example, the next time you are loading the camera pull the leader out an inch and watch it darken in direct sunlight.

In normal exposure, the latent image consists of a limited number of Ag(0) atoms within the silver halide crystal. Developers all are reducing agents that can catalise the reduction of Ag+ to Ag(0). The reaction occurs faster in the presence of Ag(0) so the silver in the crystal with the latent image speck will be reduced faster than the silver that is in a crystal that hadn't been exposed (actually the word "faster" isn't actually correct because the difference is in the probability of the reaction occurring not in the rate of the reaction). So the development of an image is due to the differential reduction of the silver around the latent image speck as opposed to other areas.

Extending this line of thinking, light can cause the formation of the image directly (aka printing out), and developer can cause the formation of Ag(0) in unexposed areas (a source of fog).

There is a whole lot more going on than this limited explaination but in a simplistic way, that is what is forming our images.

-- Fritz M. Brown (brownf@idhw.state.id.us), April 29, 1999.