..Film is on of the three elements which make photography possible. Film was developed not long after the original conception by a French Physicist, Joseph Niepce, of recording light onto an asphault like substance in a method called heliography.

In 1829, Niepce and a Parisian painter named Louis Daguerre formed a partnership to work together on improving heliography. They soon discovered a technique involving sensitizing a silver plated metal sheet with iodine fumes, exposing it to light, and developing it over mercury fumes. This was the first real developement made in getting to where modern day photography is.

Now over 165 years later, we use what is called an active emulsion layer. If we magnified a side view of some film 1000x we could see the individual light sensitive crystals suspended in a gelatin. Above these crystals is a scratch resistant coating. Below we have a film base. The film base is usually cellose-acetate in modern film types. This layer is usually coated with an anti-halation layer, designed to keep light rays from being reflected back up into the emulsion layer resulting in re-exposure of the crystals.

The most important ingredient is the silver salts, which are actually silver halid crystals, which include silver chloride, silver iodide, or silver bromide. When light penetrated the film layers, some of the silver ions in the crystals are exposed to light and are converted to metallic silver atoms. The size of these crystals can dramatically effect the speed in which this process occurs. Larger crystals usually expose faster than smaller ones. Hence we get our ISO setting actually from the size (speed) of these crystals.

There are many variables besides the speed which can change how your image will appear, but none are as significant as this constant.

The film is then placed in certain chemicals that first turn exposed crystals dark, then flush away unexposed crystals, and finally fix the transformed crystals, to make a perminent image.