Photographic film

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Undeveloped Arista black and white film, ISO 125/22°.
Undeveloped Arista black and white film, ISO 125/22°.

Photographic film is a sheet of plastic (polyester, 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 is widespread in the production of motion pictures, or movies, these are also known as films.

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[edit] 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-and-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. As indicated by Grant Haist's published book Modern Photographic Processing, B&W transparencies can be produced from most all B&W films.)

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 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 optical density, or simply density; the optical density is proportional to the logarithm of the optical transmission coefficient of the developed film. A dark image on the negative is of higher density than a more transparent image.

Most films are affected by the physics of silver grain activation (which sets a minimum amount of light required to expose a single grain) and by the statistics of random grain activation by photons. The film requires a minimum amount of light before it begins to expose, and then responds by progressive darkening over a wide dynamic range of exposure until all of the grains are exposed and the film achieves (after development) its maximum optical density.

Over the active dynamic range of most films, the density of the developed film is proportional to the logarithm of the total amount of light to which the film was exposed, so the transmission coefficient of the developed film is proportional to a power of the reciprocal of the brightness of the original exposure. This is due to the statistics of grain activation: as the film becomes progressively more exposed, each incident photon is less likely to impact a still-unexposed grain, yielding the logarithmic behavior.

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.

[edit] Film speed

Main article: Film speed

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 GOST, developed by the Russian standards authority. See the film speed article 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. In order to do this, the film must be developed for a longer amount of time or at a higher temperature 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.

[edit] History of film

Hurter & Driffield began pioneering work on the light sensitivity of film in 1876 onwards. Their work enabled the first quantitative measure of film speed to be devised.

Early photography in the form of daguerreotypes did not use film at all. Eastman Kodak developed the first flexible photographic film in 1885. This original "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. The first photographic film was made from highly flammable nitrocellulose with camphor as a plasticizer (celluloid). Beginning in the 1920s, nitrate film was replaced with cellulose acetate or "safety film". This changeover was not completed until 1933 for X-ray films (where its flammability hazard was most acute) and for motion picture film until 1951.

See also: Nitrocellulose#Nitrate film

[edit] Impact of digital photography

The development of digital photography has significantly reduced the use of film. As of 2006, film is disappearing from the consumer market except for low-end disposable cameras in western countries. This is not true of other markets, in particular the Asian market where film is still the predominant product over digital. Although many professionals have turned to digital, companies such as Kodak and Fuji have recognised the need for transparency film in the pro market and have maintained manufacturing capacity. The availability of film is also of importance to camera manufacturers.

[edit] 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- or long-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 optimized for sensing X-ray radiation is commonly used for medical imaging, and personal monitoring, and film optimized for sensing gamma rays is sometimes used for radiation dosimetry.

Film leaves much to be desired as a scientific detector: it is difficult to calibrate for photometry, it is not re-usable, it requires careful handling (including temperature and humidity control) for best calibration, and it generally requires a physical object (the film itself) to be returned to the laboratory. Nevertheless, photographic film can be made with a higher spatial resolution than any other type of imaging detector, and (because of its logarithmic response to light) has a wider dynamic range than most digital detectors. For example, Agfa 10E56 holographic film has an equivalent resolution of over 4,000 lines/mm -- equivalent to a pixel size of just 0.125 micrometres -- and an active dynamic range of over five orders of magnitude in brightness, compared to typical scientific CCDs that might have ~10 micrometre pixels and a dynamic range of three to four orders of magnitude.

[edit] Common sizes of film

See also Film format.

[edit] 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

[edit] Notable films

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  • 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 latitude. 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 it is very limited, and very little if any stock remains at photographic suppliers.

[edit] See also

[edit] References

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