Photographic film

This article is mainly concerned with still photography film. For motion picture film, please see film stock.

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

Photographic film is a sheet of plastic (polyester, PET, 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, contrast and resolution of the film. When the emulsion is sufficiently exposed 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 blocks light and appears as the black part of the film negative.

Color film uses at least three layers. Dyes, which adsorb to the surface of 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 exposed silver salts are converted to metallic silver, just as with black-and-white film. But in a color film, the by-products of the development reaction simultaneously combine with chemicals known as color couplers that are included either in the film itself or in the developer solution to form colored dyes. Because the by-products are created in direct proportion to the amount of exposure and development, the dye clouds formed are also in proportion to the exposure and development. Following development, the silver is converted back to silver salts in the bleach step. It is removed from the film in the fix step. This leaves behind only the formed color dyes, which combine to make up the colored visible image.

Newer color films, like Kodacolor II, have as many as 12 emulsion layers, with upwards of 20 different chemicals in each layer.

Due to film photography's long history of widespread use, there are now around one trillion pictures on photographic film or photographic paper in the world, enough to cover an area of around ten thousand square kilometres (4000 square miles), about half the size of Wales.

Film basics

There are several types of photographic film, including:

• 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"—that is either enlarged by projecting through a lens, or placed in direct contact as light shines through it—onto photographic paper (which in turn is itself developed) in order to be viewed as intended. Print films are available in both black-and-white and color. Color print films use an orange color correction mask to correct for unwanted dye absorptions and improve color accuracy. Although color processing is more complex and temperature-sensitive than black-and-white processing, the great popularity of color and minimal use of black-and-white prompted the design of black-and-white film which is processed in exactly the same way as a standard color film.
• 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. Conventional black-and-white negative stock can be reversal-processed, to give black & white slides, as by dr5 Chrome. Some kits were available to enable B&W reversal processing to be done by home-processors, but most are discontinued. B&W transparencies can be produced from almost 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. A simple, idealized statistical model yields the equation density = 1 - ( 1 - k) ^light, where light is proportional to the number of photons hitting a unit area of film, k is the probability of a single photon striking a grain (based on the size of the grains and how closely spaced they are), and density is the proportion of grains that where hit by at least one photon.

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 light bulb, 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

Main article: Film speed

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

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°. 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 film is that it usually has finer grain and better color rendition than fast film. Professional photographers of static subjects such as portraits or landscapes usually seek these qualities, and therefore require a tripod to stabilize the camera for a longer exposure. Photographing subjects such as rapidly moving sports or in low-light conditions, a professional will choose a faster film. 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.

History of film

Early photography in the form of daguerreotypes did not use film at all. The light-sensitive chemicals were formed on the surface of a silver-plated copper sheet. Beginning in the 1850s, thin glass plates coated with photographic emulsion became the standard medium. Although fragile and heavy, the glass used for photographic plates was of better optical quality than early transparent plastics, and in early years it was also less expensive, so the use of such plates persisted long after the introduction of film. Eastman Kodak marketed the first flexible photographic roll film in 1885, but this original "film" was actually coated on paper. After exposure, as part of the processing, the image-bearing layer was stripped from the paper base and transferred to a glass support to facilitate printing from it. The first transparent plastic film was produced in 1889. It was made from highly flammable nitrocellulose (celluloid), now commonly known as "nitrate film". Although cellulose acetate or "safety film" had been introduced by Kodak in 1908, it was not until the 1920s that it began to significantly replace the hazardous nitrate film, which had the advantages of being considerably tougher, slightly more transparent, and cheaper. The changeover was not completed for X-ray films until 1933, and although safety film was always used for 16mm and 8mm home movies, nitrate film remained standard for theatrical 35mm motion pictures until it was finally discontinued in 1951.

Spectral sensitivity

Early photographic plates and films were usefully sensitive only to blue, violet and ultraviolet light. In 1873, Hermann Wilhelm Vogel discovered that the spectral sensitivity could be extended by adding very small quantities of certain dyes to the emulsion. The instability of early sensitizing dyes and their tendency to rapidly cause fogging initially confined their use to the laboratory, but in 1883 the first commercially dye-sensitized plates appeared on the market. These early products, described as isochromatic or orthochromatic depending on the manufacturer, were also significantly sensitive to green and yellow, making possible a more accurate rendering of colored subject matter into a black-and-white image. Because they were still disproportionately sensitive to blue, the use of a yellow filter and a consequently longer exposure time were required in order to make full use of their extended sensitivity. In 1894 the Lumière Brothers introduced their Lumière Panchromatic plate, which was made sensitive, although very unequally, to all colors including red. New and improved sensitizing dyes were developed, and by 1902 the much more evenly color-sensitive Perchromo panchromatic plate was being sold by the German manufacturer Perutz. However, all of these were glass-based plate products. Panchromatic emulsions on a film base were not commercially available until the 1910s and did not come into general use until much later. In part, this was because many photographers who did their own darkroom work preferred to go without the seeming luxury of sensitivity to red—a rare color in nature and uncommon even in man-made objects—rather than be forced to abandon the traditional red darkroom safelight and process their exposed film in complete darkness. Kodak's popular Verichrome black-and-white snapshot film, introduced in 1931, remained a red-insensitive orthochromatic product until 1956, when it was replaced by Verichrome Pan. Amateur darkroom enthusiasts then had no option but to handle undeveloped film by the sense of touch alone.

Color

Experiments with color photography began almost as early as photography itself, but the three-color principle underlying all practical processes was not set forth until 1855, not demonstrated until 1861, and not generally accepted as "real" color photography until it had become an undeniable commercial reality in the early 20th century. Although color photographs of good quality were being made by the 1890s, they required special equipment, long exposures, complex printing or display procedures and highly specialized skills, so they were then exceedingly rare. The first easily used and commercially successful color "film" was the Lumière Autochrome, a glass plate product introduced in 1907. It was expensive and not sensitive enough for hand-held "snapshot" use. Film-based versions were introduced in the early 1930s and sensitivity was improved, but "color film" in the modern sense was born with the introduction of Kodachrome for home movies in 1935 and as lengths of 35mm film for still cameras in 1936. During the next several decades, color remained much more expensive than black-and-white and required much more light, factors which combined to delay its widespread adoption. Decreasing cost and increasing sensitivity gradually overcame these impediments. By the 1970s color film predominated and the use of black-and-white film was increasingly confined to low-light and "art" photography and other niche applications.

Effect on lens and equipment design

Photographic lenses and equipment are designed around the film to be used. Although the earliest photographic materials were sensitive only to the blue-violet end of the spectrum, partially color-corrected achromatic lenses were normally used, so that when the photographer brought the visually brightest yellow rays to a sharp focus, the visually dimmest but photographically most active violet rays would be correctly focused, too. The introduction of orthochromatic emulsions required the whole range of colors from yellow to blue to be brought to an adequate focus. Most plates and films described as orthochromatic or isochromatic were practically insensitive to red, so the correct focus of red light was unimportant; a red window could be used to view the frame numbers on the paper backing of roll film, as any red light which leaked around the backing would not fog the film; and red lighting could be used in darkrooms. With the introduction of panchromatic film the whole visible spectrum needed to be brought to an acceptably sharp focus. In all cases a color cast in the lens glass or faint colored reflections in the image were of no consequence as they would merely change the contrast a little. This was no longer acceptable when using color film. More highly corrected lenses for newer emulsions could be used with older emulsion types, but the converse was not true.

The progression of lens design for later emulsions is of practical importance when considering the use of old lenses, still often used on large-format equipment; a lens designed for orthochromatic film may have visible defects with a color emulsion; a lens for panchromatic film will be better but not as good as later designs.

The filters used were different for the different film types.

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.

Films can be made to record non-visible ultraviolet (UV) and infrared (IR) radiation. These films generally 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, although the infrared focal point must be compensated for.

Exposure and focusing are difficult when using UV or IR film with a camera and lens designed for visible light. The ISO standard for film speed only applies to visible light, so visual-spectrum 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. The focal point for IR is slightly farther away from the camera than visible light, and UV slightly closer; this must be compensated for when focusing. 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 by placing the subject between the film and a source of X-rays, without a lens, as if a translucent object were imaged by being placed between a light source and standard film.

Film optimized for sensing X-rays and for gamma rays is sometimes used for radiation dosimetry and personal monitoring.

Film has a number of disadvantages 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 the film must physically be returned to the laboratory and processed. Against this, 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 a resolution of over 4,000 lines/mm—equivalent to a pixel size of 0.125 micrometres—and an active dynamic range of over five orders of magnitude in brightness, compared to typical scientific CCDs that might have pixels of about 10 micrometres and a dynamic range of 3-4 orders of magnitude.

Special films are used for the long exposures required by astrophotography.

Common sizes of film

See also: Film format

• 135 — "35 mm"
• IX240 — APS (Advanced Photo System) or "Advantix"
• 110
• 126
• 127
• 120/220 — for use in medium format photography
• Sheet film — for use in large format photography
• Disc film — obsolete format used in disc system cameras
• Motion picture films — 8 mm, 16 mm, 35 mm and 70 mm

Companies

* - discontinued, ** - bankruptcy, *** - retail existing stock, production limited, **** - reseller, no manufacturing plant

• AgfaPhoto**
• Agfa-Gevaert (Materials division)
• Bergger ****
• Dai Nippon Printing
• Efke
• Foma
• Ferrania

• Fujifilm
• Ilford
• Imation (film division purchased by Ferrania)
• Kodak
• Konica Minolta*
• Lucky
• Maco****

• Mitsubishi
• ORWO
• Polaroid***
• Solaris (owned by Ferrania)
• Svema*
• Tasma

Film manufacturers commonly make film that is branded by other companies. Modern films have bar codes (DX 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

See also

• APUG
• List of photographic equipment makers
• List of photographic films
• Sensitometry

External links

• APUG (traditional photography 'analog' forums)
• British Journal of Photography
•  "Film". Collier's New Encyclopedia. 1921.