This section is from the book "Complete Self-Instructing Library Of Practical Photography", by J. B. Schriever. Also available from Amazon: Complete Self-Instructing Library Of Practical Photography.
Depth Of Focus. Depth of focus or depth of definition, in the popular meaning of those terms, is the property possessed by a lens of rendering at the same time sharp images of several objects, situated at different distances from the lens. Strictly speaking, from a purely optical point of view there can be no such thing as depth of focus; for, as was pointed out in connection with conjugate foci, any variation in distance of object means a variation in distance of image; but in practice we find that there is a more or less considerable distance within which we apparently have all the objects sharply defined. We all know that if we focus on an object very far away, everything beyond that is apparently sharp; also that, if we focus on an object a moderate distance away, we have other objects some distance nearer to and farther away from us in apparently good focus, because an image disc appears as a point when its diameter is sufficiently small; therefore when the discs which compose the different images of the different objects are within this limit, the images all appear sharp. The closer together - the more nearly in one plane the different images are, the more nearly they possess the same sharpness. The reason why all objects situated beyond a certain distance appear in equally good focus is therefore apparent. Focus with a 10 inch focus lens an object situated at a distance of 1000 times the focal length of the lens (about 833 feet). The image will then be located 1-1000 of a focal length behind the principal focus. Any and all objects situated between 833 feet and infinity will have their images all located within that distance of 1-1000 of a focal length, that is, within a distance of 1-100 of an inch. They will be very nearly in the same plane, and the diameter of the disc of confusion in all of them may be so nearly the same as to make them all appear equally sharp. For similar reasons, if we focus an object a moderate distance away, we find that other objects, both nearer and more distant, appear in good focus. Their distances relative to the object focused upon are such as to cause only a slight difference in distance of images; so slight that their disc of confusion is less than 1-100 of an inch in the plane of sharp focus for the object we selected to focus for. If we then select, one after the other, objects situated nearer to us, we find that as the object focused upon is nearer, the distance through which other objects will also appear in good focus is lessened - the depth is less, because there is a greater variation in distance of image and a consequent greater variation in the diameter of the disc of confusion, the nearer the object is to the lens. We can thus see that the farther the distance focused for, the more depth we have; the nearer that distance, the less depth, with the same lens.
Influence Of Aperture On Depth Of Focus. Apart from the distance, depth of focus depends on two factors: the aperture of the lens and the focal length of the lens. How the aperture affects the depth needs very little explanation. Every photographer knows that he can increase the depth by decreasing the aperture (stopping down), and that with a larger aperture (stop) the depth is less. The increase or decrease of the depth is proportionate to the decrease or increase of the diameter of the aperture; in other words, depth of focus varies inversely as the diameter of the aperture. Fig. 19 illustrates the influence of aperture on depth. The larger pencil (D) tapers to a point rather abruptly, and even a very slight variation in the position of our focusing screen (b) will be apt to give us a disc of more than 1-100 of an inch diameter, while the gradual tapering of the smaller pencil (d) will allow of a considerable displacement either in front of or behind the plane of greatest sharpness. Thus the one object sharply focused at (b) will appear sharp enough at (a) and at (c); and other objects so situated that their images would have their greatest sharpness at (a) or at (c), will also appear sufficiently sharp at (b).