We have exaggerated the amount of the distortion, which is slight even if the square cover almost the whole of the plate. If it cover only a small portion towards the centre it may be disregarded altogether.
It is almost unnecessary to say that when the subject is such that there are no straight lines—as, for example, in an ordinary landscape—the distortion is not perceptible.
Depth of focus.—The beginner, the very first time that he focussed a landscape or other object, will have noticed that objects whose distance from the camera is greatly different are not in focus at the same time, and that to bring a nearer object into focus he has to increase the distance between the lens and the ground glass. He will also have noticed that this difficulty of getting ob jects of various distances into focus is greatly decreased when a stop is inserted.
The quality which the stop has introduced is entitled " depth of focus." Depth of focus decreases with the aperture of a lens, and also with its focal length. No other modification (form, etc.) has any effect on it.
Width of angle.—If we cannot get far enough away from an object, but yet wish to include the whole of it in a photograph, we must include in our picture a very wide angle. Certain lenses are so constructed that they will include a very wide angle, and are called wide-angle lenses. The accompanying sketch, which shows, slightly exaggerated, the differences of form which exist between narrow and wide angle lenses, will illustrate the point.
When we want to include a wide angle we must either use a larger plate or a lens of shorter focus than if we wish to include only a narrow angle.
It may be asked, Why not use at all times the wide angle lens, and simply employ a smaller plate, if we do not wish to include all the angle which it will take in ? The reason is this :—The surface of the glasses of a lens which has to include a very wide angle are so ground that even for a narrow angle it will not give definition, unless a small stop be used, and therefore it is at best a slow lens.
Rapidity.—With different lenses the length of the ex posure necessary with the same subject and with the same light varies enormously. According as a long or short exposure is required with a certain lens, that lens is said to he slow- or rapid.
The relation between the focal length and the aperture regulates the rapidity of a lens. This we can explain
by a very simple illustration. The lens may be con sidered as lighting the ground glass or sensitive plate just as a window would light the wall of a room which is opposite it. Let us imagine a large room lighted by only one small window. We will now notice that if the size of the window were increased the wall opposite would be more brightly lighted, while if the size of the window were decreased the wall would be less brightly lighted. Here we have precisely what takes place when the aper ture of a lens is increased or decreased. There is, how ever, still another means whereby the light on the wall may be increased or decreased, besides increasing or diminishing the size of the window. Let us suppose that the wall opposite the window is not fixed, but is in the form of a movable screen. We are now quite aware that if we move the screen nearer the window it will be more brightly lighted ; if we move it away it will be less brightly lighted. If, on the other hand, we both increase the distance between the screen and the window and increase the size of the window proportionately, the amount of brightness will remain the same. Thus, if we increase the size of the window to twice its former size in each direction, and at the same time increase the dis tance between the screen and the window to twice what it was before, we shall not in any way alter the brightness.
The precise same as this takes place in the case of lenses. If we use two lenses having the same aperture, hut one of longer focus than the other—that is, involv ing a greater distance between the lens and the ground glass—it will give less light on the ground glass or sen sitive plate than with the other.
If, however, the reldion between the aperture and the focal length remain the same, the amount of light or rapidity will remain the same. For example, however different in size two lenses be, if in each the diameter of the aperture is one quarter the focal length, the rapidity will be the same in both cases. This relation between the aperture of a lens and the focal length is usually expressed thus— ' f f 4' io' These various expressions would refer to lenses in the first of which the diameter of the aperture is one-fourth the focal length, in the second of which it is one-eighth, in the third of which it is one tenth.