FOCUSING (Fr., Mise au foyer, Mise au point ; Ger., Einstellung) The action of adjusting the extension of the camera until the image is sufficiently sharply defined on the ground-glass focusing screen. A focusing magnifier assists in determining when the image is sharp by allowing slight differences in crispness to be more easily seen. As all the planes of a subject cannot be equally well defined, it is important to recognise which should show the most critical definition. Objects near the eye should naturally show greater sharpness than those farther away. But, in addition, the principal object in a picture should show the best definition, should any differences exist. Where there is a number of objects at different distances, and it is desired to secure a uniform degree of sharpness between the near est and the farthest, it is advisable to focus on a point beyond the nearest object, about one fourth of the distance from the nearest to the farthest.
In process work, several schemes have been proposed for automatic focusing. Some elabor ate mechanical inventions have been devised for moving the camera to and from the copyboard at the same time as the focusing screw is turned to move the ground glass of the camera, but none of these arrangements has come into practical use. A more convenient method, which can be applied to any camera and stand with little alteration of existing arrangements, is the Scalometer system, invented by L. Bmmett.
The scalometer is an instrument made in box wood, opening like a two-foot rule, there being on the two limbs similar scales of equal divisions numbered from the ends of the limbs. A cross rule divided into inches or centimetres bridges the angle formed by the opening of the two limbs. In operation the points or ends of the limbs are applied compass-like to the sides of the original, and then clamped at this separation. The cross rule is then made to slide up and down until it indicates across the angle formed by the limbs the measure of the desired reduction. At this point the proportional number on the two limbs is read off, and is marked on the back of the original. Then all originals which bear the same proportional number can be photographed together. By means of a printed scale supplied with the instrument it is possible to mark off the copying stand with numbers correspond ing to those on the limbs of the instrument, so that the camera can be instantly set to the proportion number marked on the original, without the necessity of focusing.