FIELD, AND DEPTH OF FOCUS "Depth of field" is sometimes used as synony mous with " depth of focus " and " depth of definition," the third expression more correctly indicating what is meant. Theoretically, objects on different planes, however small their separa tion, are brought to a focus by the lens at different points. In practice, however, it is found that there is a certain range within which objects are ren dered with a satisfactory degree of sharpness. The distance between the nearest and the farthest sharp object is the depth of definition. The two chief factors regulating this are the focal length of the lens, and the size of the stop employed ; the shorter the first and the smaller the second, the greater is the depth of definition ; the longer the focal length and the larger the aperture, the smaller is the depth of definition. If a lens is focused on a very distant object, and then slightly racked away from the screen until the limit of critical definition in the distance is reached, it will then be in the position which gives the greatest depth of definition. The nearest point showing satisfactory definition will vary according to the focal length and the stop, as already stated. The rule for finding the exact distance (known as the hyperfocal dis tance) on which to focus to secure this greatest depth, is as follows : Square the focal length of the lens (in inches), multiply by ioo, and. divide by the f number. The answer gives the hyperfocal distance (in inches). Halving this distance gives the nearest point of critical definition.
When a nearer object is focused upon there• is a certain distance both before and behind it within which the definition is also up to the standard laid down. The amount of this depth for any distauce, lens, and stop, may also be calculated. Let H be the hyperfocal distance (inches) for the given lens and stop, D the dis tance (inches) focused for. The nearest point
of critical definition is then (H x D) (H D) ; the farthest point is (H x D) — D). The range of good definition is always greater beyond than before the actual point focused upon. It follows that in estimating a distance to which the focusing scale is to be adjusted (as in hand camera work) it is better to under-estimate it than otherwise.
A lens is sometimes said to have a deep focus when it renders both near and distant objects sharply at one time ; but as the focus of a pencil of rays should be a point, it is evident that depth of focus is, strictly speaking, non-existent. In practice it is convenient to assume that an image composed of discs of confusion not more than •oi in. in diameter is " sharp " ; so that in this case the depth of definition is the distance before or behind the true focal plane between which the plate intercepts a cone of rays (of which the lens diaphragm is the base) at less than the diameter above named (•oi in.). It thus follows that the larger the working aper ture the less the depth of definition, as indicated in the diagram, in which A is the cone of rays from a small aperture, B the cone from a larger one, and c and D the diameters of the discs of confusion respectively formed ; the more acute the angle, the smaller is the disc. The surface of the sensitive film is indicated by the line E. By halving the diameter of the aperture the depth of definition is doubled, and so on in the same proportion. It also varies inversely as the square of the focal length of the lens for the same intensity, or inversely as the focal length for the same aperture. (See also" Hyper focal Distance.")