exposure index

Film stock

This focuses on motion picture film. For still photography film, see photographic film.
Film stock is photographic film on which motion pictures are shot and reproduced.

History

1889-1899

Modern motion picture film stock was first created thanks to the introduction of a transparent flexible film base material, celluloid, which was discovered and refined for photographic use thanks to the work of John Carbutt, Hannibal Goodwin, and George Eastman. Prior to this, most motion picture experiments were performed using paper roll film, which made it difficult if not impossible to view the developed film as a single continuous moving image without other complex apparatuses. Furthermore, the paper film was much more fragile than celluloid.

Eastman Kodak would become the first to make celluloid film commercially available, starting in 1889; Thomas Henry Blair emerged in 1891 as the first major competitor for supplying celluloid film. The stock had a frosted base in order to facilitate easier viewing by transmitted back light, and the emulsions from each company were orthochromatic. By November 1891 William Dickson at Edison's lab was using Blair's stock for Kinetoscope tests, and Blair's company remained a main supplier of film to the Edison lab for the next five years. Blair's operation was also crucial to the continued development of motion picture technology through 1892 and 1893, due to temporary shutdowns at Eastman because of problems with their production setup. Eventually patent lawsuits in 1893 led to Blair leaving his American company and starting again in Britain, which allowed Eastman to gradually fill the entirety of the Edison lab's film orders. Blair's new headquarters allowed him to supply many of the key European filmmaking pioneers, including Birt Acres, Robert Paul, George Albert Smith, Charles Urban, and the Lumiere Brothers. The American Blair company was to be short-lived, however, as by 1896 the new movie projector would demand a fully transparent film base that they had difficulty supplying. Eastman shortly thereafter bought the company out, thus consolidating its position as the leading supplier of film stock from then on. These developments also led Louis Lumiere to work with Victor Planchon on adapting the Lumiere "Blue Label" (Etiquette Bleue) photographic plate emulsion for use on celluloid roll film, which began in early 1896 and was brought to full production capacity by the following year.

Eastman's first motion picture film stock incorporated the same emulsion as was used for its still film, which was, like nearly all film emulsion of the time, orthochromatic-sensitive. Film at this point did not have a strictly defined speed; rather, the orthochromatic quality of the stock allowed the film to be processed under a red safelight, while the density was checked as development was occurring. Standard practice until the end of the silent era involved tearing off several inches from the start of each shot and testing development on it. Positive stock was created that would be slower, finer-grained, and of a higher contrast than a negative; all of these characteristics remain consistent to this day.

From 1895, Eastman supplied their motion picture roll film in rolls of 65 feet, while Blair's rolls were 75 feet. If longer lengths were needed, the unexposed negative rolls could be cemented in a darkroom, but this was largely undesirable by most narrative filmmakers. Actuality films were much more eager to undertake this method, however, in order to depict longer actions, and created cemented rolls as long as 1000 feet. American Mutoscope and Biograph was the first known company to use this, for the Jeffries-Sharkey fight on November 3, 1899.

1900-1919

Work on improved film stocks was rather slow in the first decade of the new century, as the equipment and formats, particularly, were still in the process of gradually standardizing. With a large number of differing film gauges in use, supplying a market still in its infancy was likely a difficult task, and Eastman was able to retain the majority of the market share without making major adjustments to the emulsion; their only major change was to increase the length of rolls to 200 feet. Lumiere, on the other hand, reformulated their stock to match the speed of Eastman film, which became known as "Violet Label" (Etiquette Violette). Blair retired back to the United States after selling his English company to Pathe in 1907, who began using the facilities for producing their own film stock. This sale had future repercussions among film preservation, as Pathe began to supplement their operation in 1910 by purchasing film prints in order to strip the silver and emulsion from the film base for recoating.

As the quantity of film and filmmakers grew in these early years, the demand for standardization increased in prominence. 35 mm film, largely thanks to the popularity of the Edison and Lumiere camera lines (and their often unauthorized clones) had begun to stabilize as the dominant gauge, but still was usually purchased unperforated, and subsequently punched by the consumer with perforation equipment designed by third parties. Although Edison (4 square perfs per frame on each side) and Lumiere (1 rounded perf per frame on each side) formats - based on the camera designs - were the most common, the perforators were not always precise, and it could be difficult to create prints for the opposite perforation format. Edison's organization of the Motion Picture Patents Trust, though largely ineffective in controlling the burgeoning film industry, was able in 1909 to agree to what would become the de facto standard: 35 mm gauge, with Edison perforations and a 1.33 aspect ratio. These parameters have remained largely constant to the present day.

The Bell and Howell company perforators entered the market in 1908 and very shortly were recognized as exceptional enough to pervade the American industry. Eastman Kodak was also quick to opt to use the machine to pre-perforate their films, which assured the perforation specifications being adopted as the world standard not long after. These perforations, known as BH-type, remain the standard for original camera negative film.

The belle epoque era also saw the creation of numerous small, local film suppliers, the vast majority of which were short-lived due to their smaller production batches, slower emulsions (which were also usually blue-only sensitive rather than orthochromatic), and inferior quality control. Among these companies, Agfa began to produce motion picture film in 1913, but remained a largely local supplier until post-World War I boycotts of popular French films allowed the newly-founded UFA film studio to flourish and thus boosted Agfa's orders.

Among the foremost problems with the film stocks of this era was that they were all manufactured on a nitrate film base, which was a derivate of guncotton and thus highly flammable. Additionally, nitrate fires were notoriously difficult to put out, as even full submersion in water will not stop the fire. This had led to a significant number of fatal accidents in theatrical projection booths, where the heat of the projector lamp made ignition most likely. As the amateur filmmaking market slowly developed at the beginning of the 20th century, Kodak began to develop a more heat-resistant "safety base" which could be easily projected without incident even at home by those with no prior experience. Early tests in 1909 showed cellulose diacetate to be a viable replacement base, and Kodak began selling acetate-base films the following year in 22 mm widths for Edison's work on the Home Kinetoscope, which was commercially released in 1912. In the wake of the availability of safety film, more amateur formats began to adopt it, and several, including Kodak's own 16 mm format, were designed specifically so that the only film stocks released for the format specifications would be safety base.

Kodak also continued to refine their camera negative stock during the late 1910s, releasing Cine Negative Film Type E in 1916 and Type F (later known as Negative Film Par Speed Type 1201) in 1917. As both of these orthochromatic films were no faster than previous offerings, it has been suggested that the improvements most likely were in regards to granularity and sharpness.

1920s

The 1920s proved to be the decade in which film stock manufacturers began to diversify their offerings. Prior to this, each manufacturer only offered one negative stock (usually orthochromatic) and one print stock. In 1920, a variant of Type F film known as X-back was created in order to counteract the effects of static electricity on the film, which can cause sparking and create odd exposure patterns on the film. This was created through the use of a resin backing on the film, which also had the effect of rendering the film too opaque to allow focusing through the back of the film - a common technique for many of the contemporary cameras of that era. As the static electricity was more likely to occur in colder climates, the X-back stock was mainly popular in the East Coast of the US. Other manufacturers also began to emerge in the 1920s as well, including American E.I. Dupont de Nemours in 1926 and Belgian Gevaert in 1925.

Conversion of the industry from orthochromatic to panchromatic stocks was initiated by Kodak over the course of the decade. First used for exterior sequences in The Last of the Mohicans in 1920 and originally only available as a special order product, the stock's increased sensitivity in the red-light range meant greater overall light sensitivity and made it an attractive option for day for night shooting. Kodak financed a feature in 1922 shot entirely with the panchromatic stock, The Headless Horseman, in order to promote the stock when it was introduced as a standard option; however, the fairly higher price of the stock compared to the orthochromatic emulsion meant that no other films would shot entirely with the panchromatic stock for several years. The cross-cutting between panchromatic and orthochromatic stocks also was noted to cause continuity problems particularly with regard to costume tones, and thus was often avoided. The dominance of orthochromatic film lasted until the mid 1920s due to Kodak's lack of competition in the panchromatic market; Gevaert emerged onto the market in 1925 with a dual product line of an orthochromatic stock with limited color sensitivity as well as a full panchromatic stock, Pan-23. This latter product likely encouraged Kodak to respond, and in 1926 they lowered the price of panchromatic stock to parity with the standard orthochromatic offering. Without any economic barrier remaining, the panchromatic stock began to overtake the orthochromatic stock's market share within a few years, as the cinematographers gradually became more familiar with the stock. With similar panchromatic offerings also made around the same period by Agfa and Pathe, the shift to panchromatic stocks had largely been completed by 1928.

Color films

Experiments with color films were made as early as the late 19th century, but practical color emulsions for amateur use were not produced until Kodak introduced Kodachrome for 16mm in 1935 and 8mm in 1936.

Before 1941, none of the color film processes for professional motion pictures used a color emulsion film stock. Kinemacolor (1908–1914), Technicolor processes 1 through 4 (1917–1954), and Cinecolor used one, two or three strips of monochrome film stock sensitized to certain primary colors or exposed behind color filters in special cameras. Technicolor introduced a color reversal stock, called Monopack, for location shooting in 1941; it was basically a 35mm version of Kodachrome that could be used in standard motion picture cameras. Eastman Kodak came out with the first 35mm color negative stock, Eastman Color Negative film 5247, in 1950. A higher quality version in 1952, Eastman Color Negative film 5248, was quickly adopted by Hollywood for color motion picture production, replacing both the expensive three-strip Technicolor process and Monopack.

Classification and properties

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 is comprised 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 chemistry 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 5500 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. Obversely, 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.

Deterioration

Motion picture film is known to be a highly unstable medium: improperly preserved film can deteriorate in a period of time much faster than many photographs or other visual presentations. Owners of home-made films often find that their film can become brittle and unwatchable in the space of a few years. Decaying acetate film stock gives off an odor similar to that of vinegar, which is why film buffs often refer to such decaying as "vinegar syndrome."

Intermediate and print stocks

The distinction between camera stocks and print stocks involves a difference in the recording process. When the work print or edit master has been approved, the original camera negative (OCN) is assembled by a negative cutter using the edited work print or EDL (edit decision list) as a guide. An answer print is then made from the cut negative. During the answer print stage, corrections in the film's density and color are corrected (timed) to the filmmakers' tastes. Interpositive (IP) prints are struck from the answerprint, checked to make sure they look the same as the custom timed answer print and each IP is then used to make one or more Dupe Negative (DN) copies. The release prints are then generated from the DN(s). Recently, with the development of digital intermediate (DI), it has become possible to completely edit, composite visual effects, and color grade the image digitally at full resolution and bit-depth. In this workflow, the answer print is generated digitally and then written out to the IP stage using a laser film printer.

Due to the specialized nature of the exposure and the higher degree of control afforded by the film lab equipment, these intermediate and release stocks are specially designed solely for these applications and are generally not feasible for camera shooting. Because intermediates only function to maintain the image information accurately across duplication, each manufacturer tends to only produce one or two different intermediate stocks. Similarly, release print stocks usually are available only in two varieties: a "normal" print or a deluxe print (on more-costly print film like Kodak Vision Premiere) with slightly greater saturation and contrast.

See also

Notes

References

External links

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