When making enlarged negatives from posi tive transparencies, the image-side of the trans parency must be turned towards the lens in all cases where the enlarged negative is to be used for printing that does not involve reversal of the image. But it is the back of the transparency that must face the lens where the negative is required for single-transfer carbon or for printing on the gelatine-coated bichromated paper of the oil process.
768. Focussing the Picture and Bringing it to the Required Size. The work of focussing the picture and bringing it to the required size is greatly facilitated if the stand of the enlarger is fitted with scales graduated in accordance with trials, supplemented, if need be, by simple calculations (§ 62). Both for focussing and for bringing the image to the right size it is well to replace the negative temporarily by a fogged negative on which inch squares have been ruled with a sharp-pointed stylus, or by a copy negative (same size) of paper ruled in inch squares. The measurement of the squares on the easel (which may be done by squares ruled on the easel) greatly simplifies the estima tion of the degree of and sharp ness is often easier to gauge in the image of a fine cut in an opaque layer than in an image with a more or less definite outline.3 When a new lens or a new light is being used for the first time, it is well to see that the sharp visual image coincides with the sharp photo graphic image. For this purpose the ruled focussing plate is enlarged on to a strip of paper arranged obliquely, e.g. at a slope of I : pp to the easel. A pencil mark is made on the line which appears sharpest, and after development the line which is sharpest is picked out and the distance between it and the mark is measured ; the displacement of the plane of sharpness will then be iiroth of the distance measured.
In choosing the scale of enlargement, remem ber that portrait enlargements termed of " natural size " must always be slightly smaller than in nature, as otherwise they will appear much too large under the usual conditions of viewing.
If the dimensions of the apparatus or of the room do not permit of the required enlargement being obtained with the usual lens, one of shorter focal length may be substituted, or the usual lens may be fitted with a supplementary con verging lens (§ 117), so as to form a system of focal length shorter than that of the usual lens.
The focussing will then have to be checked by the method described above. The aberrations introduced by such a supplementary lens fre quently have a favourable effect in reducing excessive sharpness.
Enlarging may also be done in stages, first making an enlarged transparency and then, from it, a negative enlarged to the required scale.
769. Soft-focus Enlargements. The various methods for producing images of " soft focus " (§§ 310 to 313) are applicable to enlarging, but it must be noted that the afocal supplementary lenses mentioned in § 126 can be used only with diffused light ; in the case of enlargement with directed light their effect would vary with beams of various degrees of obliquity.
One of the most commonly employed methods for modifying excessive sharpness of enlarged images consists in placing between the lens and the sensitive paper a piece of bolting silk, canvas, bolting cloth, or muslin (all preferably black). When such a fabric, stretched on a wooden frame or on a sheet of glass, is laid against the sensitive paper, it simply reduces the contrasts of the image (E. H. Farmer, 1905), giving it a pattern which, as a rule, is not objectionable. As the fabric is gradually removed from the plane of the sharp image an increased degree of diffusion is imparted to it. It is even possible to stretch one or more thicknesses of black chiffon on the lens hood, securing them by a rubber band. If the fabric is used at a considerable distance from the sensitive paper, it is necessary to place it in position before focussing, and to allow for its effect in prolonging exposure.
770. Ascertaining the Exposure. When the enlargement is made with diffused light, and in this case only, the trials for finding the correct exposure may be made with a very small strip of paper exposed in contact with the negative to be enlarged in a printing frame placed in the same position where the large sheet of paper will be placed for enlargement. This test must, of course, be made only after the various adjust ments have been carried out.