How Film Works: Silver Halide, the Latent Image and Development

A piece of film is a thin plastic base coated with gelatin containing billions of tiny light-sensitive crystals. Light changes those crystals invisibly, chemistry makes the change visible, and fixing makes it permanent. Understanding that chain makes every other part of film photography easier to reason about.

The layers of a piece of film

Film is built up in layers. The base is a clear, flexible plastic, today usually polyester or cellulose triacetate, that gives the film its strength. On top of it sits the emulsion, the light-sensitive layer, and over that a thin protective supercoat that resists scratches. Under the base, or between base and emulsion, is an anti-halation layer that absorbs light passing through the emulsion so it doesn't bounce back and spread around bright highlights.

Black and white film usually has one or two emulsion layers. Color film has at least three, each sensitive to a different part of the spectrum, plus filter layers between them.

Silver halide crystals

The emulsion is gelatin holding crystals of silver halide, mostly silver bromide with a little silver iodide. The crystals are naturally sensitive only to blue and ultraviolet light; dyes added during manufacture extend their sensitivity to green and red. Film sensitive to all visible colors is called panchromatic; film sensitive to blue and green only is orthochromatic.

Bigger crystals catch light more easily, which is the main reason faster film is grainier. Modern flat "tabular" crystals, used in films like Kodak T-Max and Ilford Delta, present a large surface to the light without a correspondingly large volume, giving more speed for less grain. The guide to film grain covers this in detail.

The latent image

When light strikes a crystal, it frees electrons that gather at sensitivity specks and reduce a few silver ions to metallic silver. Only a handful of silver atoms form, far too few to see. This invisible change is the latent image. A crystal carrying a few atoms of silver is primed to be developed; one that received no light is not.

The latent image is stable for a long time but not indefinitely. Heat and humidity degrade it, which is one reason exposed film should be developed reasonably soon, and it is the physics of the latent image that causes reciprocity failure at very long exposures.

What developer does

Developer is a reducing agent. It turns exposed crystals entirely into metallic silver, amplifying those few atoms by a factor of billions. Crystals that received no light develop far more slowly, which is why development is a matter of time and temperature: develop too long and unexposed crystals start to develop too, producing fog. The silver formed is not shiny but black, because it is made of tangled filaments that scatter light.

The more light an area received, the more crystals develop there and the darker it becomes. That is why a negative is reversed: bright parts of the scene are dense and dark on the film. The guide to developers explains how different formulas change grain, sharpness and contrast.

Stop bath and fixer

A stop bath, usually weak acetic or citric acid, halts development at the right moment. Fixer, a thiosulfate solution, then dissolves away all the undeveloped silver halide. Until the film is fixed, it is still light sensitive; after fixing, only the silver image remains and the film can be handled in daylight. Washing removes the fixer so it can't stain or degrade the image later. The full sequence is in how to develop black and white film.

How color film is different

Color film works the same way up to development, but its color developer does something extra. As it develops silver, its oxidized byproducts react with dye couplers built into each layer, forming cyan, magenta and yellow dyes exactly where silver formed. A bleach then converts the silver back to halide and a fixer removes it, leaving an image made only of dye.

Color negative film also carries an orange mask, which corrects for impurities in the dyes and is why the negatives look orange. C-41 produces color negatives; E-6 adds a reversal step to produce a positive slide.