Photographic format of film

The photographic format of film is a standardized size which makes it possible to differentiate the photographic types of existing films. It is fixed by the format of the camera, i.e. the size of the darkroom located at the back of the apparatus and which is used as receptacle with the sensitive emulsion receiving the exposure to the light. The majority of the apparatuses function with only one size of emulsion.

It is necessary also to differentiate the format from film of the format of catch of sight , even if certain film formats are standardized in order to impose a format of catch of sight (from where frequent confusion). For example, a film 135 can be used as well to take traditional images of 24x36 mm like panoramic images of 24x65 Misters.

Overall picture

The marketed formats go from small the Format 110, with the large size 8x10" (20,32 X 25,44 cm) and even 16x20" (40,64 X 50,80 cm)! The format more used since second half of the 20th century is the format 135 (still called format 35 mm or 24 X 36 mm). The Format APS of 16,7 X 30,2 mm of size of image was conceived to replace format 135, with a mitigated success.

The emulsions are either in reels of several sights (the film being rolled up around an axis inside a sheath), or in plan-film which corresponds to a single stereotype. One generally distinguishes three categories of format: the small size (110, APS and 135), the Average format (6x6, 6x7, 4,5x7, 6x9) and the large size. The two first generally come of reels (except for the back Polaroid), the plan-film last. These formats were standardized by the Norme ANSI PHI.51 1983 (“Photography Film Micrographic Sheet and Roll Film Dimensions”) then the standard ISO 1012:1998 (“Photography -- Films in sheets and films of general use --Dimensions”).

Film formats in band

The table below gives a summary of the most current formats of films in band (i.e. containing several sights) and some their associated characteristics:

Plan-film formats

One will note that a inch (noted by ") is equivalent to 25,4 mm

Influence format

As generally the whole of films profit from the last sensitometric evolutions (as opposed to what the promoters of the Format APS had claimed), the definition of the images increases with the format. Indeed, if one remains in lower part of the optical limit, the same image “is described” by all the more of money grains that the surface of the latent image is large for a given resolution of emulsion. There is thus more information in negative a 6x6 that in negative a 24x36, for the same subject and identical conditions of catch of sight.

One sees in the table above that reels 120 and 220 are declined in formats 645,6x6 and 6x7. Thus for a width of reel given, it is the camera which determines the length of the image, and in the case of the average format, there exist three standard lengths for the same width of reel. The same phenomenon exists for format 135 since certain panoramic apparatuses double the length of image and use the format 24 X 64 mm (it is nevertheless rather rare, to see in example the Hasselblad XPAN marketed in 1998 or the older Soviet Horizon).

; Various photographic formats for the same reel 120 or 220

Most of the formats above exist or existed in: Color or Black and white, Negative or Invertible (slide), and in various sensitivities. There exists even invertible black and white which make it possible to obtain slides black and white (Scala)! But it is necessary to choose between negative and slide, and color or N&B… This choice is generally of an esthetic nature (wants one to give a realistic treatment of the subject, or on the contrary to use the contrast of the black and white to dramatize the treatment of the scene, etc the newsgroups and the newspapers of photography amateur abound in this type of debates). But there are more practical aspects in this choice: speed of treatment (unfortunately the commercial laboratories guarantee fast times only for negative the color), digitalization of the tests, possibility of carrying out oneself its treatments (the black and white then has the advantage of simplicity and a greater tolerance to the variations in temperature - a black pulling and white in laboratory amateur can be carried out in a score of minutes, whereas it takes at least an hour for of the same a Ilfochrome pulling dimension. (Correction: a pulling ilfochrome takes less 20mn a pulling n&b on baryta takes much more time if one counts the time of washing)), will to project its photographs or on the contrary choice of a presentation in album (it should be noted that one can draw his slides just like his negative but the prices and times practiced by the commercial laboratories are higher).

Once this choice carried out, the differences between films is not limited to their nominal sensitivity (that marked on the box of film, generally of 25 ISO with 3200 ISO). This one is not besides really a real characteristic of film, but rather the result of tests carried out under determined conditions, which are not the tests standardized for the determination of the ISO sensitivity. For a great number of films, the manufacturers indeed prefer to rather indicate on the box an index of exposure (I.E.(internal excitation) or Exposure Index) than the ISO value, for the following reasons:

not reproducibility or non-reproduction in practice of the methods used for the determination of the ISO value (in particular for the black and white for which a great choice of methods of development exists; one gives in this case an I.E.(internal excitation) for each revealing and each temperature of development, or reciprocally),
will to increase the latitude of exposure by shifting the measurement of exposure towards zones of less great slope of the curve characteristic of the emulsion (in particular negative color),
will contrary to increasing contrast for films whose characteristic curve starts with a zone of weak slope (“software toe”),
esthetic choices of systematic under-exposure or over-exposure (into invertible a light under-exposure is supposed to produce more saturated images, an over-exposure increases the impression of brightness of the image),
possibility of to push films by under-exposing them compared to their ISO speed,
marketing choice of the manufacturer (sensitive films sold with an I.E.(internal excitation) of 3200 whereas their ISO is close to 1600).

It is nevertheless important to refer to the sensitivity ISO in order to be able to compare various films. See the Determination of the sensitivity ISO for more details.

The characteristic curve or standardized sensitometric curve: one subjects the successive zones of the same film to increasing exposures, for example each sight of a film of 36 installations with an additional quarter of diaph for each sight (or using a special device, the Sensitomètre), one develops in order to obtain the curve which makes it possible to determine the ISO sensitivity above, then one measures the density of each zone using a Densitomètre. The curve thus obtained is the curve characteristic or sensitometric curve of film. By convention, and for a better legibility, one not represents in X-coordinate the real exposure but the Decimal logarithm of this exposure (often from 0 to 4 for values which vary from 0 to 10.000).

In practice, the photographer never carries out this curve in laboratory itself but it is very useful to refer to the curves pre-established by the manufacturers and available on their sites Internet in order to compare various films. The manufacturers in general establish not the curve standardized above, but of the characteristic curves carried out with the revealing ones that they advise and at usual temperatures, i.e. generally 20 °C, 24 °C, even 30 °C. Here how to read these curves:

the minimal density, noted D_min, is the density of a negative not-talk to the light.
the threshold of emulsion, noted S, is the first value which one can distinguish from opacity due to the only support of the emulsion (or veils basic). In lower part of this value, the exposure is insufficient for sufficiently sensitizing the silver bromide grains and creating an exploitable latent image.
Starting from a point called L.D. ( Low Density = Low density) begins a rectilinear part for which there are linearity between logarithm of the exposure and density.
the concave zone located between D_min and L.D. is called the foot of the characteristic curve.
the end of the zone of linearity is marked by a point H.D. ( High Density = High density).
the maximum density is another characteristic of films. It is limited by the maximum money load per unit of area, which determines a maximum opacity. It is generally noted D_max.
the zone between H.D. and D_max is convex and is called shoulder of the characteristic curve.
the latitude of exposure: it is the difference between D_min and D_max of an emulsion. A weak latitude corresponds to a weak contrast, a strong latitude with a stronger contrast. A surer means to measure the contrast of the emulsion is to calculate gamma (γ) film, i.e. the slope of the linear segment (ΔD/Δ (logΣ).

See too

Internal bonds

silver Film
Treatment
cinematographic Format of film

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