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Photographic Images the Ideal Scientific Image the Aesthetic Image 18

luminosities, subject, prints, negative, conditions, light, obtained and viewed

PHOTOGRAPHIC IMAGES : THE IDEAL SCIENTIFIC IMAGE ; THE AESTHETIC IMAGE 18. Negative and Positive. The image obtained. by the usual photographic processes is a nega live (Fig. 2), in which the lights in the subject are reproduced as opacities and the shadows as transparencies. The photographic reproduction of this negative by a further inversion of lumin. osities gives a normal image, or positive (Fig. 3).

It cannot be too strongly impressed on a beginner not to judge the value of a negative by its appearance; the negative is only a means to an end, and should be judged only by the prints which it is capable of giving. A pretty looking negative is not always the best.

19. Range of Extreme Luminosities in a Positive. The following table indicates the ratio of the extreme luminosities in images on paper, obtained by different processes and viewed under normal conditions— Typographical impression . . from pp :1 to 35 : 131ack tone photographs, matt sur face , . . . . from i5 : 1 to 20 : Intaglio print (photogravure) . less than 35 : Carbon prints, black tone . about 40 : 131ack tone photographs, best qual .

ity glossy . 5o : P.0.1'. prints, gold toned and glazed . . . about 100 : These values should be considered as the maxima, corresponding with materials of the best quality and with perfect technique. They vary with the conditions under which prints are viewed ; an image in which the whites are more glossy than the blacks appears more contrasty when it is viewed in the open air by diffused light than in the light from a source which is almost a point. It appears still more contrasty when it is illuminated under good conditions near a window (Nutting, 194.

20. The Ideal Scientific Reproduction. In a photograph which reproduces a subject with absolute fidelity, there ought to be equality between each of the luminosities of the image and the luminosity of the subject at the corre sponding point. Obviously this equality is only possible for a certain value of the illumination of the image, and for all other values reduces to a proportionality.

Even supposing that the photographic pro cesses were able to reproduce the subject faith fully over the limited range of luminosities which can be obtained with different papers,' it can be seen that reproduction under exact conditions is impossible with an image viewed by reflection, since the range of extreme lumin osities of the subject would be limited to 20 : in the case of matt prints, or 5o in the case of glossy prints.

Note in passing the superiority, for purely record purposes, of papers with glossy surfaces, which not only allow any details to be read under considerable magnification (which cannot be done with a print the surface of which has a more or less coarse structure), but which also permit of a more correct representation of an extended range of luminosities.

Thus one is often led deliberately to depart from the ideal proportionality between the luminosities of the image and those of the subject, and to " compress " the scale of luminosities of the image in such a way as to bring it between the limits which are available in practice_ 21. The Aesthetic Image. It would obviously be correct to reproduce strictly the various tones which occur in a dark cave if the photo graph obtained was going to be used to ornament the walls of this cave, or of any other place of the same illumination. Since, however, photo graphs are usually intended to be looked at in a well-lit room, they ought therefore to ren der the physiological relations of the different luminosities of the object, and not their physical values.

The apparent relative luminosities of any scene or object change to a more or less marked degree when the intensity of the illumination of dazzling light, the artist often has recourse to the suppression or the weakening of the details in the brightest parts of the subject, whilst he conveys the sensation of obscurity to the observer by suppressing the details in the in which it is examined is modified, just as if the intensity scale were transposed into a new key (F. F. Renwick, 1918).

A lump of coal illuminated by direct sunshine can send back more light than a lump of chalk in the shade, and yet we sec the coal as black and the chalk as white. This physiological interpretation does not occur when we look at a photograph in which we may take the image of a black object for that of a white one, or con versely, according to their relative luminosities (H. Arens, 1932).

In order to give to a painting the impression shadows. These methods are based on a correct observation of Nature, and just as the artist endeavours to reproduce Nature as he sees it, so in the same way the photographer ought, with the same aim in view, to make use of know ledge derived from a study of the characteristics of the sensitive surfaces which he uses.

Such effects as the foregoing may be supple mented by others, e.g. by tinting very lightly with yellow an image representing a sunshine effect ; and with blue one which is to give the effect of night, but such general treatment must be done with extreme discretion.