| MadSci Network: Chemistry |
Hi – thanks for your question – which is a big one. To give a full answer would take much more space than we have here, so I will give some pointers and references.
Generally when processing conventional films the manufacturer will give pretty good guidelines for optimum performance. This assumes correct exposure and a reasonable dynamic range of lighting in the scene. When a photographer needs to change the effective speed of a film or if he knows he has made exposures in extreme lighting conditions there are processing tricks to help get the best result.
The effective speed of a film can be changed at exposure and the image resulting optimised by varying the process from the standard one. This is known as push or pull processing. Normally the key parameters that are varied are the development time and temperature, but other factors can be brought into play. Development concentration can be varied as can the pH of the developer. Normally, however playing with these variables is somewhat risky. Time and temperature are the usual ones and if you want to raise the speed of a film by push processing it normally involves a longer than usual development time often with a raised developer temperature. (this is probably most commonly done with reversal (slide) films). The consequences of these changes though can be things like increased grain and lowered densities in the dark areas of the image or increased minimum densities when compared to a normally exposed and processed film. There is a lot of detail available on the Eastman Kodak web site on developer variations/specifications and it would be worth having a surf around there. Here’s a link to get you started: http://www.kodak.com/US/en/motion/support/processing/h2415/h241511.shtml
You can also try a Google search on push/pull processing and see what you find.
Another fun site is http://www.spatial.maine.edu/~peggy/Teaching/Ch2_A2.ppt - if you have PowerPoint on your computer. Listen to the camera fire as you change pages.
There is no single answer to your question as what is right for a given circumstance will depend on the film, the exposure and the intent of the photographer. However processing changes operate on the characteristic curve of the film – sharpening or softening the toe – increasing contrast in the shoulder and so on. These are complex and sometimes subtle effects which you would need to read extensively to understand.
Many traditional black and white photographers “tweak” their film and paper processes to “taste” – so their technique is often unique and very empirical – not to say evolved over considerable time and practice. If you have access to a good technical library there are lots of books on the subject and a very good one is “The Manual of Photography” by Jacobson, Ray, Attridge and Axford, published by Focal Press which is in at least its 9th edition.
I hope my answer has not been too vague and that it helps you in your project – with which I wish you all the best. Have fun!
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