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Enlargements 751

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ENLARGEMENTS 751. General. A negative is enlarged either directly, by projecting on to a positive sensitive surface an enlarged image of the negative, or indirectly by first making a transparency of the same or slightly larger dimensions as the original negative, and then an enlarged negative from which the final prints are made by contact." As a rule, the direct method is chosen when only a limited number of enlarged copies of one subject are to be made on bromide paper. The indirect method is preferred for the production of a large number of identical enlargements, and it is the only practical one if the enlarged prints are required on papers of low sensitivity, such as carbon and platinum papers.

An improper application of the principle of the inverse return of luminous rays 6o) has often led to the recommendation to employ, when making an enlargement, the same lens with which the negative was taken. It is not possible to expect even the correction of dis tortion by this means, because the negative does not occupy the same position in relation to the lens that it had in the camera except when the enlarged image is of the same dimensions as the subject photographed.

While a cinematograph picture appears suffi ciently sharp in spite of its considerable magni fication, it must be remembered that what is seen is not one image but a result of several images projected in succession, in which the clumps of silver grains do not occupy the same positions ; also more attention is paid to the scene depicted than to the quality of the A photographic enlargement of about io x 8 in.

size from the standard cinema image of about x z in., i.e. an enlargement of io times' frequently gives altogether unsatisfactory re sults owing to the granularity of the images.

A well-corrected lens with which focussing has been done so as to obtain perfect sharpness such as is required in the enlarging of scientific photographs does not usually allow a degree of enlargement of more than 4 times without the graininess of the negative appearing to a disturbing extent (§ 196), particularly when a condenser is used (§ 752).

With a lens giving a slight diffusion (through incompletely correcting), or with a perfect lens to which a suitable diffusing device (§ 769) has been fitted, it is possible to obtain, especially when the negative is illuminated by diffused light, images free from granulation and quite sufficiently sharp when viewed from a normal distance, with degrees of enlargement exceed ing 15 times and even as high as 4o times.

The diffusion of the image then merges into a homogeneous medium tone the irregular tone which would have been obtained if the lens had been able to resolve the graininess of the negative. The best results are obtained, without loss of sharpness, by the use of a lens of very large relative aperture, the depth of focus of which is less than the thickness of the image layer of the negative, so that the graininess of the latter is no longer resolved. This method, which is much used for the enlargement on a very large scale of miniature negatives, is only applicable when the negative is illuminated by diffused light, for the use of a condenser would not permit of the full aperture of the lens being utilized (E. Goldberg, 1935).

752. Enlarging with Condensed and Diffused Light. Very different results, particularly as regards the contrast of the image and the prominence of minute defects in the negative, are obtained according as the negative to be enlarged is illuminated by a beam of light coming from an artificial light-source and caused to converge into the lens by an appropriate optical system (the condenser), or according as the negative is illuminated by a uniformly diffused light, coming, for instance, from a sheet of opal glass placed at some distance and illuminated by suitably distributed lamps.

Other working conditions being the same, the contrast of the image is always greater in the print enlarged with a condenser than in a print enlarged by diffused light,' and at the same time retouching and the minute defects on the negative are emphasized, owing to the increase in their contrast with the parts of the image where they appear. In an enlargement made by diffused light the contrast of the image is the same as in a contact print from the same negative on the same sensitive material. The small surface defects are not more pronounced, except as regards their size, than in the contact print.

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