EXPOSURE.
The exposure, as a rule, will be much less than for ordinary subjects, and, as the scale of intensities between light and shade is a short one, it must be given with accuracy to ensure good results. The novice cannot do better than employ one of the meters referred to in the section on Exposure, where a full ex planation of their use will be found. As planes, as shown in Fig. 796, all of which have to be more or less sharply rendered. Now the natural inclination would be to focus a point midway between these planes, under the impression that defini tion would be spread equally in both direc tions on inserting the stop. Such, how ever, is not the case. As the object draws nearer the lens, the image is formed farther back in proportion. That is to say, the point A will focus at the point B at ; but the point c will come to a focus at e, and not c, , an equal distance from The plane which focusses at will be point D, so that, if a stop is in serted, the improvement in definition will merely spread towards those two points, and the nearest point, which should he at least as sharp as any ether part, will be out of focus. If, however, a part slightly nearer than A is taken, then the points and c will both be sharply rendered. To ascertain the most effective stop to use, try them one after the other and note the effect. Supposing a lens of good quality is being employed which covers the plate a rough guide at ccmmencement, the ex posure may be the same as indicated for portraits in the tables A and B on p. 95
when the monument is photographed in a well-lit studio. Most of the plate makers now issue rough guides as to exposure and some means of testing the power of the light. No pains should be spared to obtain a correctly exposed negative, since this is a long way towards success, and saves endless work afterwards. It is best to make exposures out of doors in a good light when first starting the work, and when proficient, to attempt interior ex posures. It should be borne in mind that the exposure will in some measure depend upon the colour of the subject, and a few trial pictures may be taken of different materials, using the same plate, stop and light, and giving the same exposure. Broadly speaking, short exposures (i.e. ex posures less than the normal, or that indi cated by the meter) will give greater con trast, and longer exposures less contrast. These facts will serve as some guide, when the negative is complete, as to whether the exposure has been correct. Another very good indication is the penetration of the image through the film. For example, if the picture can be seen on the back or glass side only, it is over-exposed ; if on the film side only, it is under-exposed ; and if on both sides it is usually correctly exposed.