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MTF (modulation transfer function)

Producing an image on a photographic emulsion always involves a loss of sharpness, which is attributable to, among other things, the effects of scattered light in the emulsion. The modulation transfer function is used to plot this transfer loss in a graph. It characterizes the image sharpness of a given emulsion layer. In order to determine the MTF, a line screen consisting of thinner and thinner black lines is exposed under defined testing conditions onto the film. After processing the film – also under defined conditions – and then measuring the line screen in the micro-densitometer, we obtain the image modulation, which, because of transfer losses, is smaller than the subject modulation of the original screen. The image modulation decreases constantly with increasing spatial frequency, i.e. with an increasing number of line pairs per millimetre, until the limit of the film has been reached. The image sharpness, which declines as the lines become finer, is expressed visually in the graph of the transfer factor (see also edge effect). The transfer factor forms a curve which falls with increasing spatial frequency, and ends at the resolution limit of the film. In the case of colour films, all the colour-forming layers are tested to establish their MTF. The curves are determined by taking measurements behind separation filters, and are then evaluated jointly. The individually processed colour layers make different contributions to the sharpness of the film. Yellow contributes least to the visual impression of sharpness, while magenta contributes most. MTF data relating to a film are dependent on the measuring equipment, because the MTF of the lens system and the measuring system are included in the measurement as additional links in the transfer chain. MTF curves of films are therefore only comparable with one another if they have been corrected to take account of the MTF of the microphotometer.

Nanometre (nm).

A physical unit of length equal to 10^-9 metres, i.e. a billionth of a metre or a millionth of a millimetre. Visible light is electro-magnetic radiation in the range of about 380 to 750 nanometres.

Negative format (Advanced Photo System).

The width of the film for the Advanced Photo System is 24 mm, and the negative format is 16.7 x 30.2 mm (see: film format). The system is supplied in three film lengths with 15, 25 and 40 frames. With a negative size of 5.04 cm² the film has only 58 % of the width of the 35 mm format, but alongside the technical improvements in the emulsion and the quality features integrated in the system it has another advantage: the format makes the cameras even smaller and handier. The diagonal of the film format for the Advanced Photo System is 34 mm, and so 0.8 times the 35 mm format diagonal. To convert the focal length to the new system you therefore have to multiply by 0.8.

Fig. Comparison of the image sizes of the Advanced Photo System and the 35 mm film .

Focal length conversion table: cameras for the Advanced Photo System require shorter focal lengths than 35 mm cameras.

Printing compatibility.

One important criterion for comparing the quality of colour negative films is that films from different manufacturers – and also different film types from the same manufacturer – should produce prints of comparable quality when processed on a automatic printer with the same channel setting. One pre-condition for the printing compatibility of colour negative films is that the absorption properties of the dyes formed in the emulsion layers during processing are very similar to one another.

Paper exposure range .

The exposure range of a photographic paper is the ratio of the exposure times which are necessary to produce a defined minimum and maximum density. The ratio of the exposure times can be given arithmetically, e. g. 4 : 1; 10 :1 or 32 : 1. The international convention, however, is to use logarithms, which would be 0.6, 1.0, 1.5 in the above examples. In order to eliminate decimal figures, the logarithmic values are multiplied by the factor 100 in line with ISO Standard 6846 and prefixed with an "R" (= range). According to this standard, the above values for the exposure range would therefore be: R 60, R 100 and R 150.

Fig. : The paper exposure range is determined from the difference in the x-axis values of points s = 90 % (D MAX -D MIN ) and T (D MIN + 0.04).

The exposure range indicates what negative contrast can be reproduced by a photo paper so that details can still be recognized both in the highlights and in the shadows. Photo papers with a flat gradation (see: wide exposure range) can depict a high negative contrast, utilizing the entire grey scale from white to black. On the other hand, photographic papers with a steep gradation (see: narrow exposure range) are suitable for transforming low negative contrasts into high image contrasts.

Process compatibility.

Photographic materials are process-compatible if they can be used with the processes specified by the manufacturer and with other manufacturers' processes without any depreciation in quality. For this reason the times, temperatures and tolerances of compatible processes correspond to each other. Examples of compatible processes:

• AGFACOLOR PROCESS 70 (compatible with Process C-41) for colour negative films.
• AGFACOLOR PROCESS 94 (compatible with Process RA-4) for colour negative papers.
• AGFACOLOR PROCESS 63 (compatible with Process R-3) for colour reversal printing materials.
• AGFACOLOR PROCESS 44 (compatible with Process E-6) for colour reversal films.

Process monitoring.

Process monitoring employs certain criteria of sensitometry for the quality assurance of photographic processes. Use is made for example of exposed, un-processed control strips (sensitometer strips) on colour negative film and paper and on colour reversal film and paper, each of which are supplied to the processing labs with an exposed, correctly processed reference strip. The control and reference strips have grey test fields of different densities and coloured squares exposed onto them. The control strips are developed in the processing lab at regular intervals. The grey measuring fields on the control and reference strips are then compared visually or, for greater accuracy, evaluated using a densitometer. Big differences between the control strip and the reference strip over and above a fixed tolerance are an indication of processing fluctuations and deviations. The cause can be established from the nature of the measured discrepancy.

The steps on the control strips correspond to certain points on the density curve. Evaluation of these points provides information about the key parameters of minimum density, speed, contrast, maximum density and colour balance. In laboratory practice there is no need for an evaluation of entire curves. The processing control manuals for the various processes are an important help in fault diagnosis.

Fig. : Control strip for AGFA AP 94 process monitoring.

Reciprocity failure.

According to the reciprocity law of Bunsen and Roscoe (H = l × t = constant), it is irrelevant whether the exposure H (the product of exposure intensity and time) results from a high intensity and short exposure time (e. g. 10 lux x 1/ 1000 s), or from low intensity and correspondingly longer exposure time, e. g. 1/ 1000 lux × 10 s.

In cases of extremely long or short exposures however, the reciprocity law breaks down (depending on the emulsion used and on the wavelength of the light). This is known as reciprocity failure.

With colour films and colour printing materials, the photo-chemical process of reciprocity failure is basically the same as with black-and-white materials. With very long exposure times (studio shots with small apertures, astronomy shots, large-format enlargements), the relative speed of the materials declines (due to increased recombination of the photo-electrons with defective electrons, which form fewer latent image nuclei), while at the same time the gradation becomes steeper. Very short exposure times combined with high intensity (e. g. flash exposures) also lead to a reduction in the relative speed due to a different kind of photo-chemical loss (dispersed latent image, ultra-short-time effect). With colour materials, additional problems can arise because the three individual layers can react differently.

Very long exposures lead not only to a reduction in the general speed, but also to a possible change in the relative speeds of the individual layers. In the case of reversal films, these shifts can be corrected with the aid of colour compensating filters, and in the case of colour negative films and colour printing materials, corrections can be made by appropriate print filtering. At very low light intensities (the gradation becomes steeper) and at very high intensities (the gradation becomes flatter), the photochemical effect of reciprocity failure can differ from one layer to the next, resulting in colour deviations ( cross-cast) in areas of high and low density. These are too severe to be eliminated by correction filtering.

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