• fully measured, and this distance jotted down, as from this we
obtain the focal length of the lens. This number is multiplied
by the figure which represents the proportion of the image to
the original object, and the result is then divided by the square
of the proportional number plus one. Taking an example, we
find that the distance between the foot rule and its image equals
It must not be thought that the above proportion between the object and its image is essential; any convenient proportion may be taken, such as 4, 5, 8, etc., but the rule holds good with all.
Parallel rays proceeding from any object and transmitted by a convergent lens are refracted in such a manner that they meet at a point and form an image of that object, this point being called the principal focus. Rays which are not parallel but which diverge from an object are transmitted by a convex lens and united to a point, and the two points thus connected are said to be conjugate foci ; or in other words the distance between any point in any object and the lens, and the distance between the lens and the image of that point, are said to be the conjugate foci of the lens. These foci are of great import-ance when enlargement or reduction of any print, engraving, or negative is required. The rules for finding the conjugate focal distances are given under Enlarging (g.v.), and a table is given to save calculation.
Actinic Focus. As has been stated under chromatic aberration the actinic focus is not actually coincident with the visual focus, unless the lens be rendered achromatic.
Depth of Focus is the power of defining upon a plane surface with sufficient definition to satisfy the requirements of the case, the images of objects situated at varying distances. Theoretically depth of focus is an impossibility, but practically when any point is focussed sharply there is a certain distance before and behind that point which is also sharp. To find this distance the fol-lowing rul# may be used. The use of diaphragms increases it; the smaller the aperture the greater the depth of focus. Having focussed any point, to find the distance in front of that point