I built a setup for digitiziation with a DSLR to process my old negatives some of which have a sentimental value.
Since I do not have a macro lens, I combined an SLR lens with a couple of cheap adaptors. The rest was improvised from pieces of wood, cardboard , PVC and even meccano parts.
Step 1: About film negatives
- Film photography is based on the sensitivity of silver halides (AgBr, AgI etc) to light. The upper layer of the film is a coating of gelatin containing crystal grains of silver halides.
- When the film is exposed to light, some Ag+ ions are excited to higher energy states and a few combine with electrons to form Ag which will act as nucleation centres in the development stage. At this point the latent image is formed .
- When the film is developed with chemicals, an oxidation - reduction reaction takes place. The silver ions in the exposed regions are reduced to neutral silver atoms which coagulate to form metal grains. This is a negative image since the film becomes non transparent in the exposed regions.
- Colour photography is based on a variation of this process with the addition of organic coloured dyes coupled to the silver halides. The film coating has three layers for the three basic colours (see photo).
- Black and white negative images consist of silver grains which are stable over time. The supporting gelatin is sensitive to humidity and temperature but if stored properly B/W film can last for hundreds of years.
- On the other hand the colour negatives are more sensitive to environmental factors because of the organic dyes they contain. The cyan dyes fade away faster and the negative becomes reddish. Exposure to light causes the magenta dyes to fade also, due to ultraviolet light.