Photographic film

Photographic film consists of a transparent base, either a strip or sheet, coated with a gelatin emulsion. This emulsion holds microscopic silver halide crystals that react to light. The size and shape of these crystals control the film’s sensitivity, contrast, and resolution. The film is usually divided into frames, each producing a separate photograph.

If left in the light, the emulsion slowly darkens, but this process is too gradual and incomplete for practical photography. Instead, a camera lens provides a very brief exposure, creating a slight chemical change in each crystal proportional to the light it absorbs. This produces an invisible latent image, which can later be chemically developed into a visible photograph.

Most films respond not only to visible light but also to ultraviolet light, X-rays, gamma rays, and high-energy particles. Unmodified silver halide crystals only detect blue light, which makes some colored subjects look unnatural. This was solved by adding sensitizing dyes that attach to the crystals, allowing them to react to other colors. First came orthochromatic film, sensitive to blue and green, and later panchromatic film, sensitive to all visible colors. Panchromatic film renders colors in shades of gray that roughly match their perceived brightness. Similar techniques can make special-purpose films sensitive to infrared light.

Black-and-white film typically has a single layer of silver halide crystals. During development, exposed crystals turn into metallic silver, which blocks light and appears black on the negative. Color film uses at least three sensitive layers, each with different sensitizing dyes. The top layer is blue-sensitive, followed by a yellow filter that stops blue light from reaching the lower layers. Below that are a green-and-blue sensitive layer and a red-and-blue sensitive layer, which record green and red images. During development, exposed silver halide crystals become metallic silver, just like in black-and-white film. However, in color film, by-products of this reaction combine with chemicals called color couplers—either in the film or the developer—to form colored dyes. The amount of dye is proportional to the exposure and development. After development, the silver is turned back into silver halide crystals in a bleach step, then removed during fixing with ammonium thiosulfate or sodium thiosulfate (hypo or fixer). This leaves only the color dyes, which form the visible image. Later color films, such as Kodacolor II, contain up to 12 emulsion layers, each with over 20 different chemicals.

Photographic film and motion picture film stock are similar in composition and speed, but often differ in frame size and length. Silver halide photographic paper is also similar to film. Before digital photography, film had to be developed to create negatives or projectable slides, and negatives were printed as positive images, usually enlarged. This was typically done by labs, though many amateurs processed their own film.

**Characteristics of film**

**Film basics**

Several types of photographic film exist. Print film, when developed, yields transparent negatives with inverted light, dark, and colors (if color film is used). This film is designed to be printed onto photographic paper, usually with an enlarger or by contact printing. The paper is then developed, and the second inversion restores normal light, shade, and color. Color negatives include an orange color-correction mask that compensates for unwanted dye absorptions, improving print accuracy. Although color processing is more complex and temperature-sensitive than black-and-white processing, the wide availability of commercial color labs and the scarcity of black-and-white services led to some black-and-white films that can be processed exactly like standard color film.

Color reversal film produces positive transparencies, also called diapositives. These can be viewed with a magnifying loupe and a lightbox. When mounted in small frames for slide projectors or viewers, they are commonly called slides. Reversal film is often sold as “slide film.” Large-format color reversal sheet film is used by some professionals to create very-high-resolution images for digital scanning into color separations for mass reproduction. Photographic prints can be made from reversal film transparencies, but direct positive-to-positive print materials (like Ektachrome paper or Cibachrome/Ilfochrome) have all been discontinued. This now requires an internegative to convert the positive transparency into a negative, which is then printed as a positive.

Black-and-white reversal film exists but is very rare. Conventional black-and-white negative film can be reversal-processed to produce black-and-white slides, for example using dr5 Chrome. Although chemical kits for this process are available, they are not common.