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lens, table and distance

CONJUGATE FOCI.—A lens has two foci having a definite relation to each other, and are, therefore, termed the " conjugate foci." See Focus.

Besides the equivalent focus of the lens, which should be known by the worker, it is often useful to know—ist, the size of the image that will be obtained; 2d, the distance between the object and the camera; 3d, the size of the object to be photographed; and 4th, the distance of the focusing screen from the center of the lens. All this information can be obtained from the following table of conjugate foci, compiled from Suter's metrical tables by Mr. J. R. Gotz.

To explain the table, the top line of Table I. contains the equivalent foci of different lenses from 3 to 36 inches, and the relative proportions of the images and the object itself are given in the vertical column at the left hand side. The other figures are the distance that the object is from the lens, and the distance of the lens from the ground glass focusing screen.

If it is required to reduce with a lens of a given focal length, follow the horizontal line from the figure denoting the proportion required until the column headed by the equivalent focus of the lens used. If the tables be reversed they may be used for enlargement.

Table II. gives the approximate sizes of the image of a figure of a man 5 feet 9 inches from life size to a reduction of As an example, a head 3 inches would be a third of its natural size (taken at 9 inches). If a lens at is-inch focus be used it will, according to the table, require a distance of 5 feet, the posterior focal distance being 20 inches.

Another instance of the use of a table of this kind: If a building, say, 25 feet high and 25 feet wide, is to be taken at a distance of 4o feet with a 6-inch lens, the reduction will be, according to the table, and the image will, therefore, occupy about 4% inches horizontally and 3 inches vertically, and would require a half-plate camera to get a good picture.