How Photographic Film Works

What does it really mean when you “take” a picture with a camera? When you click the shutter, you have frozen a moment in time by recording the visible light reflected from the objects in the camera’s field of view. In order to do that, the reflected light causes a chemical change to the photographic film inside the camera. The chemical record is very stable, and can be subsequently developed.

If you were to open a 35-mm cartridge of colour print film, you would find a long strip of plastic that has coatings on each side. The heart of the film is called the base, and it starts as a transparent plastic material (celluloid) that is 4 thousandths to 7 thousandths of an inch (0.025 mm) thick. The backside of the film (usually shiny) has various coatings that are important to the physical handling of the film in manufacture and in processing.

There may be 20 or more individual layers coated here that are collectively less than one thousandth of an inch thick. A very special binder that holds the imaging components together takes up the majority of this thickness. It is a marvellous, and ubiquitous material called gelatine. A specially purified version of edible gelatine is used for photography — yes, the same thing that makes Jell-O jiggly holds film together, and has done so for more than 100 years! Gelatine comes from animal hides and bones. Thus, there is an important link between a cow, a hamburger and a roll of film that you might not have appreciated.

Colour print paper is a high-quality paper that is specially made for this application. It is made waterproof by extruding plastic layers on both sides. The face side is then coated with light-sensitive silver-halide grains that are spectrally sensitized to red, green and blue light. Since the exposure conditions for a colour print paper are carefully controlled, the paper’s layer structure is much simpler than that of the colour negative film. Once again, gelatine plays a key part as the primary binder that holds the image-forming grains and the colour-forming components (couplers) together in very thin, individual layers on the paper surface.



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