Photographic film

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Image:Undeveloped film.png

Photographic film is 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 and resolution of the film. When the emulsion is subjected to sufficient 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.

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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) inverted. This type of film must be "printed" — either projected through a lens or placed in contact — to photographic paper in order to be viewed as intended. Print films are available in both black & white and color.
  • 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. (Conventional black and white negative stock can be reversal- processed, to give 'black & white slides', and kits are available to enable this to be done by home-processors - however, the gamma required for an effective slide is high, and more easily achieved with a slower film like Pan-F).

In order to produce a usable image, the film needs to be exposed properly. The amount of exposure variation that a given film can tolerate while still producing an acceptable level of quality is called its exposure latitude. Color print film generally has greater exposure latitude than other types of film. Additionally, because color print film must be printed to be viewed, after-the-fact corrections for imperfect exposure are possible 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 parts of the image are exposed heavily enough to approach the maximum density possible for a print film, then they will begin losing the ability to show tonal variations in the final print. Usually those areas will be deemed to be overexposed and will appear as featureless white on the print. Some subject matter is tolerant of very heavy exposure; brilliant light sources like a bright lightbulb, or the sun, included in the image generally appear best as a featureless white on the print.

Likewise, if part of an image receives less than the beginning threshold level of exposure, which depends upon the film's sensitivity to light - or speed - the film there will have no appreciable image density, and will appear on the print as a 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 threshold sensitivity to light. The international standard for rating film speed is the ISO scale which combines both the ASA speed and the DIN speed in the format ASA/DIN. Using ISO convention film with an ASA speed of 400 would be labeled 400/27°. ASA is by far the more popular of the available standards, especially with newer equipment, and is often used interchangeably with the term ISO, although DIN retains popularity in Germany. The prevalence of ASA is reflected in film packaging which normally boldly states the ASA speed of the film on the box, with the full ISO speed printed in smaller type on the reverse or base. A fourth naming standard is the GOST developed by the Russian standards authority. See film speed for a table of conversions between ASA, DIN, and GOST film speeds.

Common film speeds include ISO 25, 50, 64, 100, 160, 200, 400, 800, 1600, and 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", as it requires much more exposure to produce a usable image than "fast" ISO 800 film. Films of ISO 800 and greater are thus better suited to low-light situations and action shots (where the short exposure time limits the total light received). 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 and the slower the film.

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 photographers who do their own development or professional-level photofinishers. More rarely, a film can be pulled to behave like a "slower" film.

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History of film

Pioneering work on the light sensitivity of films was done by Hurter & Driffield from 1876 onwards; this work enabled the first quantitative measure of film speed to be devised.

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.

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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. Infrared films may be shot in standard cameras using an infrared band-pass filter.

Exposure and focusing are also difficult when using UV or IR film with a regular camera and lens. The ISO standard for film speed only applies to visible light, so regular light meters are nearly useless. Film manufacturers can supply suggested equivalent film speeds under different conditions, and recommend heavy bracketing. e.g with a certain filter, assume ISO 25 under daylight and ISO 64 under tungsten lighting. This allows a light meter to be used to estimate an exposure. For focusing, the focal point for IR is slightly father away from the camera than visible light, and UV slightly closer. Apochromatic lenses are sometimes recommended due to their improved focusing across the spectrum.

Film sensitized to X-ray radiation is commonly used for medical imaging, and personal monitoring.

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Common sizes of film

See also Film format.

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Companies that manufacture photographic film

Film manufacturers commonly make film that is branded by other companies. Modern films have bar codes on the edge of the film which can be read by a bar code reader. This is because film is sometimes processed differently according to specifications of the film, determined by its manufacturer; the bar code is entered into the computer printer before the film is printed.

To establish the OEM, read the bar code printed on the cassette. Divide the long number by 16 and record the number before the decimal, then multiply the number after the decimal by 16, this could give you a result such as 18 and 2.

The first number is known as the PRODUCT (film manufacturer) and the second number as the MULTIPLIER (speed of the film ISO). In the previous example, 18 identifies 3M as the manufacturer and 2 means it is 200 ISO:

  • 3M = 18
  • Agfa = 17 or 49
  • Kodak = 80, 81, 82 or 88
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Notable films

  • Kodak Kodachrome is one of the oldest slide films still being produced and is known for its long archive stability.
  • Fuji Velvia, also a slide film, is known for its high contrast and hyper-saturated colours. It is popular with landscape and nature photographers.
  • Both Kodak T-max p3200 and Ilford Delta 3200 are B&W films with very wide exposure lattitude. They are rated at roughly ISO 1000, but can be pushed to ISO 3200 or higher. Rated speeds of as high as ISO 25,000 have been obtained.
  • Kodak Technical Pan, which has now been discontinued, is a widely acclaimed slow black and white film. With a speed of ISO 25, it gave clear, incredibly fine-grained results. It has now become somewhat of a commodity item among photographers as very limited, if any stock remains at photographic suppliers.
  • Maco IR 820c / Rollei IR are one of the two last remaining infrared films. After Konica's discontinuation of their IR 750 film and taking into accound Kodak HIE's uncertain future, this infrared film is the only one that is certain to remain in the near future. It is very slow and is sensitive to infrared light of up to about 820 nm in wavelength.
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See also

es:Película fotográfica gl:Película fotográfica ko:필름 it:Pellicola fotografica he:סרט צילום ja:フィルム nn:Fotografisk film pl:Błona fotograficzna ru:Фотоплёнка simple:Photographic film sv:Film (foto) th:ฟิล์ม tr:Film (fotoğrafçılık) zh:底片

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