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Exposure range.

The exposure range is the ratio between the lowest and the highest light intensity of an original (e. g. a grey step wedge) at a constant exposure time. The figures are given arithmetically (e. g. 1 : 32) or, as in the AGFA Technical Data Sheets, logarithmically, e. g. 1.5 lg ( logarithms).

Film base (Advanced Photo System).

The film base is made of PEN (Polyethylene Napthalate). It is 30 % thinner than the conventional 35 mm material, but has a much higher tensile strength.

Film format (Advanced Photo System).

A feature of the film, which is available with 15, 25 and 40 frames, is the smaller negative (16 × 30 mm). In addition the continuous perforation on conventional films has been replaced by two registration holes per negative.

Film format . Advanced Photo System (APS) .

Gradation.

The gradation describes the rendition of contrast of a photographic material compared with the original. The gamma (γ ) describes the gradient or angle of slope of the straight section of a density curve with respect to the exposure axis. A material with a flat gradation (i. e. a flat gradient) turns high contrasts in the original into low contrasts in the image. Conversely, a material with steep gradation (i. e. with a steep gradient) changes low contrasts in the original into high contrasts in the image. There is a connection between the gradation and the exposure latitude of a photographic material (at a given maximum density). A material with flat gradation permits a greater exposure latitude, while a material with steep gradation has a correspondingly smaller exposure latitude. The exposure latitude of the material should at least correspond to the luminance range of the subject. In addition to this, it should also provide a reserve against over-and under-exposure. This is why it is a particular advantage for photographic materials to have a wide exposure latitude. Negative materials have a wider exposure latitude than reversal materials due to their gradation. Typical gradation values for various photographic materials are given below.

Black-and-white materials (BW)

BW film approx. γ0.7 BW paper γ1.3 -3.8

Colour negative materials (CN)

CN film approx. γ0.65 CN paper γ1.5 -2.0

Colour reversal materials (CR)

CR film approx. γ1.4

CR paper approx. γ1.0

Duplicating film

approx. 1.0

Inter-negative film

approx. γ0.5 (variable from 0.3 to 1.2 according to exposure)

Black-and-white and colour negative films generally have a relatively flat gradation and a wide exposure latitude with a γ value of 0.65 -0.7. Negative paper has a much steeper gradation than the film. The approximate gradation of a print is calculated with the Goldberg rule, as the product of film contrast and paper contrast:

γprint = γfilm × γpaper

In the colour negative/ positive system, the image gradation is for example:

γCN film × γCN paper = 0.7 × 2.0 = γ1.4

The much steeper image gradation of colour reversal film (it corresponds to the image gradation = γ 1.4 of the colour negative/ positive system) means that the exposure latitude is smaller than that of the CN film.

Fig. 12: The slope of 45° of the straight section of the density curve represents a γ value (tan ) = 1.

The gradation of a paper print produced from a slide is obtained as follows using the Goldberg rule:

γslide × γCR paper = 1.4 × 1.0 = γ 1.4 or

γslide × γinternegative × γCN paper = 1.4 × 0.5 × 2.0 = γ1.4.

With black-and-white photo papers, the distinction between fixed-contrast and variable-contrast papers is very significant. Whereas a fixed-contrast paper (e. g. AGFA BROVIRA SPEED) will need to be available in up to five different grades (including e. g. soft, normal and hard grades), one single variable contrast black-and-white paper can produce different contrast grades by colour filtering of the printing light ( variable contrast papers).

Granularity .

The term grain is generally taken to mean a single silver halide crystal. The sum of all the grains in the various light-sensitive layers makes up the image-forming structure. The visual impression of this system, which can be evaluated with the aid of measuring techniques, is called the granularity.

By granularity we mean the irregular texture of a uniformly exposed and developed film surface. This irregular structure comes about through local density fluctuations, which can be measured with a microphotometer.

Comparisons of granularity are carried out, for example, using the root mean square method ( RMS). The granularity data given in the AGFA Technical Data Sheets relate to an RMS value measured at a net density of 1.0 with a visual filter. This value is then multiplied by the factor 1000. The diameter of the measuring aperture should, wherever possible, be selected so that the most important conditions for visual observation and the optical properties of the eye are simulated.

Granularity Representation of thin sections through the emulsion coating taken with an electron microscope.

Fig. 13: The micro-structure of a developed photographic emulsion.

Grating spectrograph.

The spectral sensitivity of a photographic material is established by exposing the film through a narrow slit in the grating spectrograph using light of a given energy distribution. Depending on the sensitivity ( speed) of the black-and-white or colour material, a black-and-white or coloured density profile is obtained (depending on the wavelength) after development, which reflects the spectral sensitivity of the black-and-white or colour emulsion layers under examination. The density profile can be measured with a densitometer. In order to enable the measurements to be compared quantitatively, the energy distribution of the lamp spectrum is found and the resultant spectral sensitivity converted to an equi-energy spectrum.

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