Photographic film

Photographic film consists of a transparent base, usually a strip or sheet, coated on one side with a gelatin emulsion. This emulsion holds microscopic silver halide crystals that are sensitive to light. The size and shape of these crystals control the film’s sensitivity, contrast, and resolution. The film is divided into frames, each frame producing a separate photograph. If left exposed to light for a long time, the emulsion gradually darkens, but this process is too slow and incomplete for practical photography. Instead, a camera lens provides a very brief exposure, causing a slight chemical change in each crystal proportional to the amount of light it absorbs. This creates 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 are sensitive only to blue light, which makes colored subjects look unnatural. This was solved by adding sensitizing dyes that adhere to the crystals, allowing them to react to other colors. First came orthochromatic film, sensitive to blue and green, and then panchromatic film, sensitive to all visible colors. Panchromatic film renders colors in shades of gray that roughly match their subjective 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 are converted into metallic silver, which blocks light and forms the dark areas of the negative. Color film uses at least three sensitive layers, each with different sensitizing dyes. The top layer is sensitive to blue, followed by a yellow filter that prevents blue light from reaching the layers below. Below that are layers sensitive to green-and-blue and red-and-blue, which record green and red images. During development, exposed silver halide crystals become metallic silver, just as 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 in the developer—to form colored dyes. The amount of dye formed is proportional to the exposure and development. After development, the silver is converted back to silver halide crystals in a bleach step, then removed during fixing with a solution of ammonium thiosulfate or sodium thiosulfate. This leaves only the color dyes, which create the visible image. Later color films, such as Kodacolor II, contain as many as 12 emulsion layers, each with up to 20 different chemicals. Photographic film and 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 produce negatives or projectable slides, and negatives were printed as positive images, usually enlarged. This was typically done by photographic laboratories, 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 color values. This film is designed to be printed onto photographic paper, usually with an enlarger or by contact printing, and the paper is then developed. The second inversion restores normal appearance. Color negatives include an orange color correction mask that compensates for unwanted dye absorptions and improves print accuracy. Although color processing is more complex and temperature-sensitive than black-and-white processing, the wide availability of commercial color processing and scarcity of black-and-white services led to the design of 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 lightbox. When mounted in small frames for use in a slide projector or viewer, they are commonly called slides. Reversal film is often marketed as “slide film.” Large-format color reversal sheet film is used by some professional photographers 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 (such as 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 uncommon. Conventional black-and-white negative film can be reversal-processed to produce black-and-white slides, for example by dr5 Chrome.