Photographic film
Photographic film is a light-sensitive recording medium used by an analog camera. The term derives from the original meaning of the word "film" as a thin layer over another material. It is a transparent surface, in most cases flexible, covered with a thin layer of photographic emulsion made of gelatin, into which a light-sensitive substance such as silver bromide is introduced.
Film cartridges that go into cameras are called rolls or reels.
Process
Originally, the photosensitive layer was made of celluloid, but today it is made of cellulose acetate or other plastics such as polyester. The most modern are made of silver halides, with a variable particle size (granularity) that affects the sensitivity of the film and the characteristics of the final image. When this emulsion is subjected to a controlled exposure of light or other types of rays —generally through a set of lenses (objective)—, the image is very faintly imprinted on the film, receiving the name of latent image. In order to obtain an unalterable image in future exposures to light —the photographic image or photographic snapshot—, a series of chemical processes are applied to the film, in a process called photographic development, which amplifies the existing image and stabilizes the image. Photographic film is, in turn, the basis for the photographic process known as chemical photography, the conventional process for creating images, the predecessor of digital photography.
Black-and-white photography uses a single layer of silver halide emulsion, while color film uses a minimum of three layers.
Types of films and uses
Perhaps one of the most critical decisions when taking a photograph is the choice of film. How to know which, among all the available films, is the one that will best suit our needs? To answer this question, we must know the characteristics of the different photosensitive emulsions available on the market, and for what purposes they have been developed.
A first division of photographic films is made based on the type of emulsion, and therefore the type of image that is obtained after developing it. According to this criterion, the films can be:
- Negative in black and white (B/N) (which in turn can be orthochromic or panchromatic), you get an image in reverse grey tones of the original, that is, the lights of the motif are dark or black greys and the shadows of the motif are light grey or white in the negative. The movie orthochromatic is sensitive to the entire spectrum except the red color. The movie panchromatic is sensitive to all the colors of the visible spectrum.
- Negative in color, you get an image in the complementary colors of the originals.
- Negative Infrared in black and white, registers the wavelengths between 700 and 1200 spectrum nanometers by using an IR filter in front of the target which prevents the entry of ultraviolet radiation and visible spectrum radiation (in total or partial form). They are characteristic shades of white for green, black for blues, and variations of gray for other colors.
- Slide in black and white (almost in disuse), you get in the movie an image in gray tones of the same value as in the original motif, that is, the lights correspond to light and white greys while the shadows are dark grey and black
- Slide in color, you get an image with the same colors as those of the original motive.
- Infrared tax is a film sensitive to green, red and infrared. For this reason, the surfaces that emit infrared radiation are reproduced in red, while the blue is reproduced as such, although with greater intensity. Originally designed for military purposes has varied applications in scientific photography.
- Film lith or online movie, it is a very high-contrast negative film, because it only reproduces black and white. It has an interest in the reproduction of documents, screen printing and the development of slides with schematics or graphics, as well as their application in creative photography.
Color films (both negatives and slides) have an additional characteristic, that is, their white balance.
What we call white light is nothing more than the result of adding the different wavelengths that make up the visible spectrum. Sunlight does not produce the same white as candlelight. The latter, due to its temperature, has a greater amount of radiation in the red band, so the result is a warmer light. Actually, it is our brain that interprets a certain light as white regardless of whether its source is the blue of the sky, a fluorescent light or a tungsten lamp. But objectively, each of these sources has a different color temperature, which is expressed in kelvins (K).
The photographic emulsion is not capable of interpreting colors as our brain does, so it will be impressed by the combination of colors that actually exist as a result of the wavelengths that each body reflects or emits. Therefore, if the white light source is the blue sky, the photograph will tend to appear bluish, while if the light source is a lamp it will appear orange. To avoid this effect, color films are balanced at a specific color temperature, that is, they are calibrated for a certain white from which we will obtain the entire tonal range.
Clearly producing film for each of the different possible lighting sources would not be possible or cost-effective, so the photographic industry has simplified it to the two most common types of lighting:
Daylight films balanced at 5,600 K which is the average temperature of sunlight at noon.
Tungsten light films, that is, for lighting with photographic lamps, balanced at 3,200 K if it is type A and 3,400 K if it is type B. In general, this is the type of film used when shooting indoors without a flash and the light source is an incandescent bulb.
For all other photographic situations, the following color corrective filters can be used, depending on the film to be used:
Daylight film with tungsten illumination Filter 80A Daylight film with halogen illumination Filter 80B Daylight film with fluorescent lighting Magenta filter Tungsten Light Film B with Daylight Filter 85B
Photographic emulsions can also be classified according to a new parameter. This is sensitivity. The sensitivity of a photographic film is the amount of light with which its photosensitive emulsion reacts. Some photographic brands speak of E.I., this is Exposure Index or Exposure Index.
The exposure index or sensitivity of a film is indicated by a photographic sensitivity scale. There are different scales: ASA, DIN, ISO or GOST (Soviet scale currently in disuse).
Whether an emulsion is more or less sensitive depends on the size of the photosensitive halide grains. Therefore, when the size of the grains is large, the area occupied by each particle is greater, so a smaller number of photons incident against the emulsion will be enough to produce the latent image, with which the result will be a time of shorter exposure.
Films are classified according to their sensitivity as follows:
for low sensitivity (or slow movies): from ISO 6 to ISO 64. They have extremely fine grain and a very wide tonal scale. They allow large enlargements to be made without the grain being perceptible. These films are used when great detail is required in the image, with static objects and when there is good lighting or when long exposures with a tripod are possible. Ultra slow films, usually ISO 6 are those used for contact copying.
Of medium sensitivity: from ISO 100 to ISO 200. They are usually considered all-terrain sensitivities, which is why they are the most used. They have a wide tonal scale and allow enlargements up to 30cm x 40cm with barely noticeable grain.
for high sensitivity (or fast movies): from ISO 400 to ISO 3200. They have low contrast. The grain is coarse and evident in enlargements, so the image loses definition. They are used in action photographs where it is required to freeze the movement or in low light situations.
We can affirm as a general rule that the higher the sensitivity of the film, the lower its contrast will be, but there are special cases where the emulsion has been designed for specific purposes where both the way in which they produce densities and the color of the base of celluloid affect the final contrast of the print (in the case of low-contrast films, they may have yellow supports and reach low maximum densities, high-contrast films may have blue supports and reach high maximum densities).
Another important concept associated with films is the exposure latitude, or what is the same, the margin of error in the exposure that an emulsion allows, giving acceptable results. The most sensitive films have greater latitude than the less sensitive ones. Regarding the different emulsions, the ones with the greatest latitude are the black and white film negatives (up to 2 stops in some cases), followed by the color negatives (1 stop maximum), with the lowest latitude being the slides with barely 1/2 and even 1/3 diaphragm. The result is that, using slide, the photographer has very comprehensive control of color saturation with the camera controls without the need to enter the darkroom, but any error in the calculation of the exposure will spoil the shot. On the contrary, the use of negative or B/N film allows a certain flexibility in the calculation of the exposure in the shot.
When choosing a film it is essential to know what use we are going to give to the image we obtain. In many cases, and in a perfectly controlled light situation, this will be the primary discriminatory factor when selecting an emulsion.
The negative film or B/W will basically serve us to obtain a positive on paper, useful for illustrating works and publications. The main advantage of the B/W negative is its ease of processing in a home laboratory, which translates into its immediacy. On the other hand, during printing (and without having a professional laboratory) a large number of manipulations can be carried out, from selective framing to removing backgrounds, which allows obtaining images of enormous visual impact.
From a color negative it is possible to obtain copies on paper both in color and in B/W, it is also possible to obtain slides, but it is an expensive process and not always of a satisfactory quality. Its development is relatively simple, however, printing in a home laboratory is much less comfortable, since in addition to having to work in almost complete darkness, it must be added that the slightest variations in the temperatures of the developing liquids cause serious alterations. color, both negative and positive.
The slide can be projected, so it is useful in conferences and classes, it allows you to obtain high-quality paper copies of it, both in color and in B/W, you can make duplicates of them easily, and it is the photographic material that printers require to produce quality color reproductions. Its development does not present special difficulties, as long as we have a rigorous control of the temperature. Finally, it could be added that they are easy to file and transport.
Companies that develop photographic films
- Agfa
- Adox
- Efke
- Foma
- Forte
- Ferrania
- Fujifilm
- Ilford Photo
- Kodak
- Konica
- Lomography
- Maco
- Orwo
- Perutz
- Polaroid
- Rollei
- Lucky.
- Valca
- Svema
- Kentmere
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