Photographic Materials and Processing Chemistry
This section covers Photographic Materials and Processing Chemistry: film and paper structure, latent-image formation, the developer-stop-fix sequence, colour chemistry, temperature control and faults in negatives and prints.
Core rules
- Black-and-white film has a base and a gelatin emulsion containing silver halide crystals. Exposure forms a latent image; developer reduces exposed crystals to metallic silver; stop bath, dilute acetic acid, halts development; fixer, sodium or ammonium thiosulfate, dissolves unused halide; washing removes residues; wetting agent prevents drying marks.
- Colour negative film uses dye couplers and the C-41 process: colour developer CD-4, bleach, fix or bleach-fix, stabiliser, around 38 °C with tight time and temperature control. Colour slide film uses E-6; colour paper uses RA-4.
- More exposure or development raises density; less gives thin negatives. Contrast depends on development time and paper grade.
- Grain increases with film speed. Push processing raises effective speed but increases contrast and grain.
- Safelights: black-and-white paper can use amber or red; panchromatic film and colour paper need total darkness or the specified safelight.
Worked example
A negative is thin and lacks shadow detail. Reasoning: shadows received too little exposure, so few silver halide crystals were exposed. Extending development would increase density in highlights already exposed and raise contrast, but would not create shadow information. The correct fix is more camera exposure through a lower shutter speed, wider aperture or higher ISO film, not longer development. This separates exposure faults from development faults.
Traps
- Swapping stop bath and fixer; fixing first can leave undeveloped silver and stain.
- Using a red safelight with panchromatic film or colour paper.
- Ignoring developer temperature; colour processes are far less tolerant than black-and-white.
- Diagnosing all density problems as development when the shutter or meter caused underexposure.