THE CHOICE OF NEGATIVE.
The essential qualities in a negative for enlarging are, (a) extreme sharpness of definition, not merely in the centre of the image, but all over the picture ; (7)) clear ness of shadows ; (e) proportionate grada tion in both high lights and shadows ; (d) freedom as far as possible from all mechanical defects such as pinholes, spots, etc., which are, of course, magni fied and rendered more apparent in direct proportion to the degree of enlargement. To secure quality (a) it is advisable to use a somewhat smaller stop than is custom ary for general work, also to exercise especial care in focussing. When it is impossible to use a small stop, as in groups or certain other work necessitating instantaneous exposures, the composition should be as near as possible in the same plane. It frequently happens, however, that such arrangement is impracticable, and then recourse may sometimes be bad to swinging the back so as to accommodate the focus ; the angle at which it is swung must be such that the line passes through the focus of the principal objects. This position may be ascertained beforehand by calculation or by trial. If, however, the background or the subject contains any vertical parallel lines, the rendering will be false, and it then becomes a matter of compromise. To secure quality (h) a slight extra trace of potassium bromide may be added to the developer, and development should not be allowed to pro ceed quite so far. Much, however, will
depend upon the brand of plate, sonic makes allowing considerably more develop ment than others, while preserving I he clearness of the shadows. The addition of the potassium bromide will have the further advantage of securing a better gradation in the high lights, if develop ment clues not proceed so far as to clog them and produce too much contrast. A dense deposit in the high lights will lead to large bald patches in the picture, which can only be dealt with with the greatest difficulty. It is useless having gradations in the high lights which are out of propor tion to the shadows. As pointed out in the section on Exposure, all attempts to force out detail in one portion cannot but result in a weakening of all the other gradations in the picture. Resort must then be had to shading parts in the en largement, which, though feasible, be comes at times very complicated. In all cases as little shading and dodging as possible is advisable, as, although it is surprising what may be done by a worker of skill and experience, yet it is usually attended with considerable risk of failure. Negatives which have been retouched to any great extent also need special treat ment.