There are several variables in classifying stocks; in practice, one orders raw stock by a code number, based on desired sensitivity to light.
Base A piece of film consists of a light-sensitive
emulsion applied to a tough, transparent
base, sometimes attached to
anti-halation backing or "rem-jet" layer (now only on camera films). Originally the highly flammable
cellulose nitrate was used. In the 1930s, film manufacturers introduced "
safety film" with a cellulose triacetate plastic base. All amateur film stocks were safety film, but the use of nitrate persisted for professional releases.
Kodak discontinued the manufacture of nitrate base in 1951, and the industry transitioned entirely to safety film in 1951 in the
United States and by 1955 internationally. Since the late 1990s, almost all release prints have used
polyester film stock.
Emulsion The emulsion consists of
silver halide grains suspended in a
gelatin colloid; in the case of color film, there are three layers of silver halide, which are mixed with
color couplers and interlayers that filter specific light spectra. These end up creating yellow,
cyan, and
magenta layers in the negative after development.
Chemistry Development chemicals applied to an appropriate film can produce either a positive (showing the same densities and colors as the subject) or
negative image (with dark highlights, light shadows, and, in principle, complementary colors). The first films were darkened by light: negative films. Later films that produce a positive image became known as
reversal films; processed transparent film of this type can be
projected onto a screen. Negative images need to be transferred onto photographic paper or other substrate which reverses the image again, producing a final positive image. Creating a positive image from a negative film can also be done by
scanning the negative to create a computer file which can then be reversed by
software.
Image record Different emulsions and development processes exist for a variety of image recording possibilities: the two most common of which are black and white, and color. However, there are also variant types, such as
infrared film (in black and white or
false color); specialist technical films, such as those used for
X-rays; and obsolete processes, such as
orthochromatic film. Generally, however, the vast majority of stock used today is "normal" (visible spectrum) color, although "normal" black and white also commands a significant minority percentage.
Physical characteristics Film is also classified according to its
gauge and the arrangement of its
perforations— gauges range from
8 mm to
70 mm or more, while perforations may vary in shape, pitch, and positioning. The film is also distinguished by how it is wound with regard to perforations and base or emulsion side, as well as whether it is packaged around a core, a daylight spool, or within a cartridge. Depending on the manufacturing processes and camera equipment, lengths can vary anywhere from 25 to 2000 feet. Common lengths include 25 feet for 8 mm, 50 feet for
Super 8, 100 and 400 feet for 16 mm, 400 and 1000 feet for 35 mm, and 1000 for 65/70 mm.
Responsivity A critical property of a stock is its
film speed, determined by
ASA or its sensitivity to light listed by a measurement on the
raw stock which must be chosen with care. Speed determines the range of lighting conditions under which the film can be shot, and is related to
granularity and contrast, which influence the look of the image. The stock manufacturer will usually give an exposure index (EI) number equal to the ASA which they recommend exposing for. However, factors such as forced or non-standard development (such as
bleach bypass or
cross processing), compensation for filters or
shutter angle, as well as intended under- and over-exposure may cause the cinematographer to actually "rate" the stock differently from the EI. This new rating is not a change to the stock itself — it is merely a way of calculating exposure without figuring out the compensation after each light reading.
Color temperature Another important quality of color film stock in particular is its
color balance, which is defined by the
color temperature at which it accurately records white.
Tungsten lighting is defined at 3200 K, which is considered "warmer" in tone and shifted towards orange;
daylight is defined at 5600 K, which is considered "colder" and shifted towards blue. This means that unfiltered tungsten stock will look normal shot under tungsten lights, but blue if shot during daylight. Conversely, daylight stock shot in daylight will look normal, but orange if shot under tungsten lights. Color temperature issues such as these can be compensated for by other factors such as lens filters and color gels placed in front of the lights. The color temperature of a film stock is generally indicated next to the film speed number — e.g. 500T stock is color film stock with an ASA of 500 and balanced for tungsten light; 250D would have an ASA of 250 and be balanced for daylight. While black-and-white film has no color temperature itself, the silver halide grains themselves tend to be slightly more responsive to blue light, and therefore will have daylight and tungsten speeds — e.g. Kodak's Double-X stock is rated 250D/200T, since the tungsten light will give slightly less exposure than an equivalent amount of daylight.
Sound A fundamental limitation of film stock as a recording medium is that it reacts to light, but not sound. This is why the first films were
literally silent (and exhibitors often provided live musical accompaniment to compensate).
Sound films later became possible after engineers developed techniques like
sound-on-disc to synchronize playback of a separate soundtrack and then
sound-on-film to print the soundtrack on the film itself. ==Deterioration==