Analogic photography was for a long time the only way to produce images. Before digital imaging, the film was the support analog image production for over 150 years. Photography is originally the product of a reaction between light and a chemical support. In the 16th century, we already knew the light action on silver chloride but it was not until 1802 that the Englishman Thomas Wedgwood published his method. In 1819, Sir John Herschel, son of the famous astronomer and discoverer future infrared rays improves the Wedgwood method.

At the same time, the French Niepce tackles the same problem and gets his first "héliographics drawings." He partnered with a scene-painter concerned about the iconographic reproductive issues, Jacques Daguerre, but he died too soon to appear in 1838, the first "daguerreotypes." The first negative and positive first date of 1839 (the English W.H.Fox Talbot). The first photographic roll of celluloid in 1889 (American George Eastman). As to the first color plate look-through in a commercially viable process has been developed in 1904 by the Lumière brothers. Note that the first photograph in 1827 had requested the eight hours exposure time during which the sun had time to illuminate both sides of buildings.

Processing a film

Process a film is to make usable the support after the shooting. Five steps lead to attach images from latent negative to negative readable.

1 - Transfer of the film in a completely light-tight vessel
2 - Processing and revelation at 20 ° C with water, developer and fixer
3 - The stop bath of the developer action at pH 3.5 to 5.5 with a water with 10% acetic acid or 30% white vinegar
4 - The bath fixer leading to the stabilization of the negative image with an active agent called thiosulfate
5 - Fuser cleaning for stopping its action
6 - The drying of the film up to 2 hours on a wire in a place without dusts

General film composition

Main film characteristics

Two films types and packaging

A film is composed of silver nitrate grains. Each grain is very sensitive to light and is capable of storing light information. In films, it must however differentiate two types : silver color and silver Black & White supports because they do not really react the same way. Nevertheless, they all have many elements in common :

  • The support
  • The light-sensitive element.
  • Annexes layers (anti-halo anti-curl, anti-abrasion)

The film resembles the composition of a printed circuit board with its three annexes layers :

  • The abrasion-resistant layer protects the film mechanical attack.
  • The antihalation layer absorbing the light reflected by the support in the emulsion. This stray light affects the image resolution and contrast.
  • The anti-curl layer preventing the film curve.

The use of a film is determined by its characteristics. They influence the final result of the event as well as its use terms. The main features are seven in number :

  • The rapidity. I.S.O indicates the emulsion sensitivity to light.
  • The granularity. It refers to the size silver metal agglomerates which form the image. More rapid is the film and more granularity is important.
  • The acutance or microcontraste.
  • The exposure latitude. It is the ability of the emulsion to render the extremes luminance of the subject.
  • The contrast or "gamma". It expresses the emulsion contrast as a function of process.
  • The sensitivity spectrum. It gives the relative sensitivity of the emulsion to different spectrum colors.
  • The Schwarzschild effect or gap in the reciprocity law. Unlike digital imaging, the performance of the film is not constant as and as exposure time lengthens. More long exposed to light and must be exposed !

Analogic photography can be practiced with a negative film or a reversal film (slide). The film conditioned in cartridge (film wound in coil several views) or on a film plane for a single shot. The most common format is 24x36 packaged in cartridges with a 135 DX code allowing the device to read different information about the film (sensitivity, number of shots and exposure latitude).

The DX code was in the form of twelve bare or painted metal boxes on the cartridge body 135. The boxes are read by a series of contacts 12 in the film unit receptacle. The paint acts as insulation as if there was or not conductivity of these 12 contacts, the camera was then able to "guess" himself the film type to print.

Analogic photography concepts

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OBSERVATION AND IMAGERY

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Arnaud FIOCRET
 

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