The photographic film (or film ) is a flexible support covered with an emulsion containing of the compounds sensitive to the light, generally containing halides of money. Their configuration (size and the shape of the crystals in particular) determines the characteristics of film like the Sensibilité and the definition.
When the emulsion is subjected to an exposure to the light in a camera, it is formed a latent, invisible image. It is necessary to obtain a visible image to proceed to the development, a chemical process in several phases.
the films used by the Cinema and the photograph are rather close; this article treats however only those intended to carry out photographs.
The film
The support
The support must be resistant and flexible at the same time, with a low coefficient of elongation, a good resistance to the ageing and of course a perfect compatibility with the emulsion.It receives, before the emulsion itself a " layer; anti-halo" who avoids a reflection light at the time of the catch of sight. Once the emulsion deposited, a anti-stripes layer protects the whole.
The current films have a support of triacétate of cellulose for films in rollers or Polyester for the plan-films, thick of approximately 0,1 Misters.
Various formats
- dedicated Article: photographic Format of film
Cartridges and reels
Cartridges 135 carry a Code DX making it possible the apparatus to read various information on the film: sensitivity, many sights and latitude of exposure. At the end of the exposure, the film must be re-coiled inside the opaque cartridge with the light.A contrario , reels 120 and 220 is rolled up on a take-up reel; their opaque protection over the entire length (120) or finale (220) protects them from the light with the proviso of not forgetting the adhesive strip!
Types of emulsion
Monochromatic " Black and Blanc"
Silver films
It is directly silver which forms the negative image.- panchromatic Film: sensitive to all the extent of the visible spectrum. It is the type of film " by défaut" used in photography.
- orthochromatic Film: far from sensitive to the red. Used in reproduction.
Film chromogene
Manufactured according to the principle of the emulsions color, therefore with couplers; it develops besides with the same C41 process.Colors
- Invertible or positive: give slides
- Négatif: is then drawn on paper
Balance colors
As well into invertible as into negative, beside the films most current, known as " light of the jour" , exist films for artificial light balanced for the Température of color of 3400 K of the incandescent lamps.
Special
- Infra-red monochromic
- Infra-red colors: image false colors
Principles of operation
Negative film black and white
the black film and white consists of a layer of protection, crystals of halide of money incorporated in the gelatin, of transparent plastic, anti-halation backing - against the iridescent aureole which occurs around the lines of a stereotype when the subject presents contrasts violent one!
Negative film color
It makes it possible to obtain after development a negative film which will have to be drawn on sensitized paper to give a photograph.Its basic principle is the superposition of three layers of emulsions sensitive respectively to the three primary colors: blue , separated by a layer filters yellow layers green and red . The yellow filter blocks the blue component of the light. Each of the three principal layers comprises silver bromide grains sensitive to the light and a coupler which will make it possible to form a dye during the development chromogene. At the time of this one, each coupler reacts near the exposed money grains and releases a complementary dye of color. These dyes are however insufficiently selective and " polluent" returned other layers:
- the layer sensitized with blue is coloured in yellow
- the layer sensitized with the green is coloured in magenta with a yellow secondary
- the layer sensitized with the red colors in cyan with secondaries magenta and yellow
- with the " layer; vert" a yellow mask
- with the " layer; rouge" a magenta mask and yellow are, into subtractive, red
The layers filters and anti-halation become transparent during the development.
The most recent films comprise a complementary layer sensitive to the magenta cyan becoming clear for one made optimal of the soft nuances in particular of the skin.
Positive film color - slide
The positive film color makes it possible to directly obtain an observable photograph by transparency or projection, the slide.The development of the slides is made according to processes specific to the invertible films.
In the general case of the slides of the type Ektachrome (E6 treatment), each of the three principal layers comprises grains sensitive to the light which form an negative image. This one is reversed by chemical veil before making act the couplers which will form the dye during the development chromogene:
- the layer sensitized with blue is coloured in yellow
- the layer sensitized with the green is coloured in magenta
- the layer sensitized with the red is coloured in cyan
In the particular case of the Kodachrome, the process is more complex with a treatment sleep by layer (inversion then contribution of couplers).
Characteristics of film
Sensitivity
The sensitivity of a film is its capacity with being impressed by a more or less great deal of light. The more sensitive one film is, the more the exposure could be reduced.The sensitivity is indicated by a value in ISO which associates old units ASA and DIN. ASA is still used in the current language: a film of " 400 ASA" mean ISO 400.
The ISO values are proportional to the sensitivity: a film ISO 200 is twice more sensitive than a film ISO 100. The value of Sensibilité ISO of films available varies from 25 to 1600 color, up to 3200 in N&B. The standard films have a sensitivity between 100 and 400.
Detailed characteristics
Smoothness of the grain: with equal technology, plus the emulsion is sensitive, plus the grain is important. Its sensitometric curves (one by color) determine the Contraste and the Saturation obtained and make it possible to obtain an optimal exposure.The Pouvoir solving is the aptitude of the emulsion of film to separate the finest details. It is measured in line pairs per mm and is function of the contrast of the image. It usually exceeds 100 pairs of lignes/mm in the recent emulsions color and 200 in N&B. The most relevant measurement of this resolvent capacity is that of the answer in space Fréquence or transfer function transfer of modulation (FTM).
Returned colors depends on the quality of the spectral response of each of the 3 layers and their balance.
Treatment
The silver treatment calls upon chemical processes, influenced by physical parameters: temperature, concentration of the reagents and reaction time.Each type of film (positive or negative, black & white or color) follows a different process with important nuances according to the manufacturer and the generation.
See the articles: Development of films black and white, Development of a negative film color, invertible Film (or slide color).
A little history
They were manufactured in nitrate of cellulose, very dangerous because highly flammable even explosive and, in any case, unstable and breaking up into spite of all the precautions of conservation.George Eastman used it towards 1888 to produce the first flexible photo film for a simple case, the Kodak " 100 vues". It improves it towards 1900 to allow the daylight loading and introduces it into the " Brownie n°1" who popularized photography near the general public.
See too
- Sensitivity Standard ISO
- of silver emulsions
- Effect Schwarzschild: sensitivity according to the luminosity
- the wikilivre of photography (in the course of drafting)
Simple: Photographic film
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