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DIN, ASA and ISO speeds .

Film material for taking photographs is divided into various speed categories. A distinction is made between slow, medium, fast and ultra-fast films. Different systems are used for stating the film speed: the arithmetic ASA system and the logarithmic DIN system. Doubling or halving the photographic speed is equivalent to doubling or halving the ASA number in the ASA system, or raising or lowering the DIN number by three units in the DIN system. Both speed systems are combined in the international ISO standard:

There is also an international standard for stating the speed of black-and-white photo papers (ISO 6846).

This standard is an arithmetic speed value, in other words doubling or halving the figure corresponds to doubling or halving the speed. To distinguish it from the ISO standard used for films, the speed designations for black-and-white photo papers have a letter P in front of the figure, for example ISO P 400.

DIN speed.

This is the absolute measure of the speed of black-and-white negative films. In line with DIN 4512/ 1, the DIN speed is determined at a medium contrast and a medium luminance ratio of the subject (see also DIN, ASA and ISO speeds .)

DIR couplers.

The DIR couplers used in modern colour negative films produce much improved colour saturation, a finer grain and better sharpness compared with conventional films. The first colour film with built-in colour couplers was launched in 1936 (the AGFACOLOR NEU). The next technical breakthrough came in the seventies with the introduction of the DIR couplers. DIR stands for Development Inhibitor Releasing. These couplers release development-inhibiting compounds ( inhibitors), more in areas of higher density than in areas of lower density (in proportion to the exposure and developer activity). The inhibitors diffuse in all directions in the film layers, are adsorbed by developing silver halide crystals and inhibit their further development. The coupler can be either colourless or coloured.

The DIR couplers of the first generation improved one property in particular: by preventing complete development of adjacent silver halides (partial reduction of the silver halide), a finer grain was produced than in conventional processes without DIR couplers, even though relatively large silver halide crystals were used. DIR couplers of the latest generation also specifically improve the sharpness and colour rendition. The inhibitors diffuse within the emulsion layer (= horizontal inhibition) from points of high density (where development is inhibited to a lesser extent) to points of low density (where the development inhibition is greater). The higher micro-contrasts obtained in this way increase the impression of sharpness ( edge effect; transfer factor).

The effects of modern DIR couplers on the colour rendition are even more pronounced. The inhibitors diffuse from layers of high density into neighbouring layers of lower density (= vertical inhibition, also known as interimage effect). As a result, the colour density of the dominant colour layer is intensified, whereas in the adjacent layer of low density dye formation is suppressed. In this way, undesirable secondary densities (impurities of the main colour density) can be compensated (e. g. the yellow and cyan secondary densities of the magenta dye). The net result is greater colour purity.

DX code.

All the major film manufacturers now make use of the DX coding system. It is an electronically readable information system for 35 mm films.

The DX system comprises the following components. Checkered code on the film cartridge: This electronically readable code enables DX cameras to automatically detect the film speed, exposure latitude and number of exposures. The checkered code consists of two rows of electronically conductive (silver) and non-conductive (black) squares.

Bar code and numerical code on the film cartridge.

The electronically readable cartridge bar code provides processing information for automatic identification of the manufacturer, the process, the film type, the film speed and film length. In the photo-finishing labs, it permits automatic sorting for the specified process. Between the film slot and the bar code is a visually readable numerical code with the same content as the bar code.

Information area on the film cartridge.

The film type, film length and speed are indicated on a particular area of the film cartridge. Suitably equipped cameras display this information in the inspection window.

Bar code at the edge of the film.

AGFACOLOR films have a digital bar code exposed onto them at the edge of the film below the perforations. Photofinishing labs use this information for automatic product recognition.

Fig. 8 : DX coding on the film cartridge.

Edge effect .

The rendition of the light-to-dark transition of an adge ( for example of a grid ) exposed on to a photographic material characterizes the sharpness, i.e. the edge or contour sharpness of that material . Due to the diffusion processes ( scatering of the inciden light by the silver halide crystals ), light also gets into the shadow area of the adge, resulting in a weakening of the edge, and consequently a reduction in sharpness (the decrease in sharpness also becomes visible as a drop in the modulation transfer function á MTF curve). Minimizing light scatter by the silver halide crystals during exposure (by using thinner emulsion layers and applying less silver) brings an improvement in overall sharpness. The edge effect can also be used to improve the impression of sharpness.

Fig. 9: The edge effect is based on exaggerating the light-to-dark transition lines.

Concentration differences and chemical diffusion effects occur at light/dark contours during processing. What happens is that reaction products – so-called inhibitors (e.g. bromide) – are released during film development, and these diffuse from areas of high density (in which a lot of inhibitor is released) into areas of lower density (less inhibitor). The net result is an intensification of the light-to-dark transition lines, i.e. a local density increase in the more strongly exposed part and a density decrease in the less strongly exposed part of the edges. Areas of low density thus have a seam of lower density before the transition to darker areas. Areas of high density, on the other hand, have a seam of higher density before the transition to lighter areas. The resultant visual improvement in contour sharpness is also documented by a more favourable MTF curve (a MTF, a transfer factor): the increase in visual sharpness can be so marked that the MTF values exceed 100 %. Modern colour films with their high speed and excellent image quality would simply not exist if we did not take advantage of these chemical inhibition effects. The same applies to the outstanding sharpness of modern black-and-white films.

MTF curve for AGFAPAN APX 100

Fig. 10: MTF curve for AGFAPAN APX 100. The high sharpness of the film (MTF values sometimes above 100 ) is due among other things to the edge effect.

Modern colour films with their high speed and excellent image quality would simply not exist if we did not take advantage of such chemical inhibition effects. The same applies to the outstanding sharpness of modern black-and- white films.

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