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Photographic film

Photographic film a sheet of plastic (polyester, celluloid (nitrocellulose) or cellulose acetate) coated with an emulsion containing light-sensitive silver halide salts (bonded by gelatin) with variable crystal sizes that determine the sensitivity or resolution of the film. When the emulsion is subjected to controlled exposure to light (or other forms of electromagnetic radiation such as X-rays), it forms a latent (invisible) image. Chemical processes can then be applied to the film to create a visible image, in a process called film developing.

In black-and-white photographic film there is usually one layer of silver salts. When the exposed grains are developed, the silver salts are converted to metallic silver, which block light and appear as the black part of the film negative.

Color film uses at least three layers. Dyes added to the silver salts make the crystals sensitive to different colors. Typically the blue-sensitive layer is on top, followed by the green and red layers. During development, the silver salts are converted to metallic silver, as with black and white film. The by-products of this reaction form colored dyes. The silver is converted back to silver salts in the bleach step of development. It is removed from the film in the fix step. Some films, like Kodacolor II, have as many as 12 emulsion layers, with upwards of 20 different chemicals in each layer.

Because photographic film was ubiquitous in the production of motion pictures, or movies, these are also known as films.

Undeveloped Arista black and white film,  125.
Undeveloped Arista black and white film, ISO 125.

 

Film basics

There are two primary types of photographic film:

  • Print film, when developed, turns into a negative with the colors (or black and white values, in black and white film) inversed. This type of film must be "printed" (projected through a lens) to photographic paper in order to be viewed as intended. Print films are available in both black & white and color.
  • Reversal film after development is called a transparency and can be viewed directly using a loupe or projector. Reversal film mounted with plastic or cardboard for projection is often called a slide. It is also often marketed as "slide" film. This type of film is often used to produce digital scans or color separations for mass-market printing. Photographic prints can be produced from reversal film, but the process is expensive and not as simple as that for print film. Black and white reversal film exists, but is uncommon one of the reasons reversal films are popular among professional photographers is the fact that they are generally superior to print films with regards to color reproduction.

In order to produce a usable image, the film needs to be exposed properly. The range of tones that a given film can accurately record is called its exposure latitude. Color print film generally has better exposure latitude than other types of film. Additionally, because color print film must be printed to be viewed, some after-the-fact correction of the exposure can be made during the printing process.

The concentration of dyes or silver salts remaining on the film after development is referred to as density. A dark image on the negative is of higher "density" than a more transparent image. If part of the image exceeds the maximum density possible for a print film, then it is overexposed and will appear as featureless white on the print. Likewise, if part of an image is beneath the minimum density possible on a film, the area will appear as featureless black. Some photographers use their knowledge of these limits to determine the optimum exposure for a photograph; for one example, see the Zone system. Most automatic cameras instead try to achieve a particular average density.

Film speed describes a film's overall sensitivity to light. The international standard for rating film speed is the ISO scale. Common film speeds include ISO 25, ISO 50, ISO 100, IS0 200, ISO 400, ISO 800, ISO 1600, and ISO 3200. Consumer print films are usually in the ISO 100 to ISO 800 range. Some films, like Kodak's Technical Pan, are not ISO rated and therefore careful examination of the film's properties must be made by the photographer before exposure and development.

ISO 25 film is very "slow", so it requires much more exposure to produce a usable image than ISO 800 film. Films of ISO 800 and greater (referred to as "fast" films) are thus better suited to low-light situations and action shots. The benefit of slower films is that it usually has finer grain and better colour rendition than fast film. Professional photographers usually seek these qualities, and therefore require a tripod to stabilize the camera for a longer exposure. Grain size refers to the size of the silver crystals in the emulsion. The smaller the crystals, the finer the detail in the photo.

A film with a particular ISO rating can be pushed to behave like a film with a higher ISO that is, exposed for a shorter period of time than would normally be used. In order to do this, the film must be developed for a longer amount of time than usual. This procedure is usually only performed by the photographer who does their own development, or by professional-level photofinishers. More rarely, a film can be pulled to behave like a "slower" film.

History of film

The first flexible photographic film was made by Eastman Kodak in 1885. This "film" was coated on paper. The first transparent plastic film was produced in 1889. Before this, glass photographic plates were used, which were far more expensive and cumbersome, albeit also of better quality. Early photography in the form of daguerreotypes did not use film at all.

Special films

Instant photography, as popularised by Polaroid, uses a special type of camera and film that automates and integrates development, without the need of further equipment or chemicals. This process is carried out immediately after exposure, as opposed to regular film, which is developed afterwards and requires additional chemicals. See instant film.

Specialty films exist for recording non-visible portions of the electromagnetic spectrum. These films are usually designed to record either ultraviolet or infrared light. These films can require special equipment; for example, most photographic lenses are made of glass and will therefore filter out most ultraviolet light. Instead, expensive lenses made of quartz must be used.

Film sensitized to X-ray radiation is commonly used for medical imaging.

Common sizes of film

See also Film format.

Companies that manufacture photographic film

See also


 

 

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