In this case, and as a rough first indication,' the optimum exposures for the enlargement of a given negative with a given light, a given sensitive paper, a given lens always used with a given stop, are proportional to the values (G ± i)2, G representing the degree of enlarge ment. If, for instance, a given negative is successively enlarged 2, 3, 4, 5, etc., times, the optimum exposures will be in the proportion to the numbers 9, i6, 25, 36, etc.
This method of calculation fails with directed light, for the flow of light transmitted by the condenser varies with the adjustment of the distance of the and if the light is of comparatively large size its image may be larger than the surface of the lens aperture.
In this case the trials for ascertaining the exposure must of necessity be done on a strip of paper placed on the easel and illuminated by the enlarged image in the same conditions under which the final print will be made. 3 When the exposures best suited for the contact printing of two negatives of very different densities on identical sensitive papers are known, all operations being carried out under the same conditions, then the proportion of these two exposures will hold good in the case of enlarge ments of the same magnification in diffused light and under identical conditions (same light, same paper, same development).
The optimum exposure can also be ascer tained, in the actual conditions of enlarging, by measuring the illumination of the shadows of the projected image with a luxmeter or any equiva lent simplified device, or by means of a photo electric exposure meter, provided that these instruments are calibrated by methodical trials for this special purpose.
771. The determination of the optimum exposure with daylight is more difficult owing to the frequent variations in the intensity of the light. It is best done by the use of two identical negatives, e.g. the halves of a stereo negative, but separate.
A strip of sensitive paper is exposed in the enlarger in sections, and the optimum time corresponding to the working conditions (char acter of the negative, sensitiveness of the paper, intensity of the light) is ascertained. At the same time a piece of print-out paper is exposed under the second negative in a printing frame facing in the same direction as the enlarger, and the time required to obtain a weak image, but with all detail, is noted. The proportion between the optimum exposure of the strip and the exposure of the print-out will always remain unaltered, whatever the negative used, provided that the working conditions are the same (same degree of enlargement, same lens aperture), and the same two papers are used. If two identical
negatives are not available, the trials will have to be made successively with one negative, choosing the middle of the day in clear weather.
It is sometimes possible to adjust the aperture of the stop of the enlarger so that these two exposure times are the same. The time of exposure of the enlargement will then be equal to the printing time of a print-out paper with a similar negative. 1 772. Exposure. Everything having been ar ranged, the light from the enlarger is cut off by capping the lens so as to permit of the sensitive paper being placed in position. Usually a cap fitted with an orange or red glass is used, as this permits of the image being seen and of the paper being placed correctly in position. The lens is then uncapped for the time found neces sary for exposure. It is then re-capped to allow of the removal of the paper.
If the negative has not been provided with a mask, cutting off all the parts not required in the enlargement, it is necessary to make sure, before putting the paper in position, that no light object is in the path of the beam in such a position as to reflect or diffuse light on to the sensitive paper.
For enlargements by daylight the apparatus must be pointed towards the sky, care being taken that external objects (projecting cornices, trees, adjoining buildings) do not come in the field of view.
In the use of daylight enlargers or of triple body cameras, the sensitive paper is placed in position by means of a dark slide, and the exposure is made by hand with a shutter. Some cheap patterns are not fitted with a shutter, the exposure being made with a piece of cardboard covering the negative carrier.
773. Control During Exposure. In all cases where the image is projected on to sensitive paper exposed uncovered in the dark-room, it is easy to modify the tones of the various parts of the image (particularly when a long exposure is given) by masking the parts to be lightened for a suitable fraction of the exposure. The mask used must always be held fairly well away from the sensitive paper, so as to project on it only a softly vignetted outline, and it must be kept in constant movement to soften the edges of the cast shadow still further.