OPTICS.
The very first time that any object was focussed it will have been observed that the nearer the object was to the camera the greater was the distance required between the lens and the ground glass to make the object sharp. In other words, the focus of a lens is longer when a near object is focussed than when a dis tant one is. Now, if we consider almost any subject which we are likely to photograph, it will be evident that there are in it different parts which are at different distances from the camera. Could we so arrange our ground glass and our sensitive films that those portions which received the impressions from distant parts of the subject were nearer the lens than those which received the impression of near parts, we might have everything in sharp focus even with the largest aperture of any lens. Now something approaching this can be brought about in certain cases. If we have either at the top or bottom of our subject a portion which is nearer the camera than the rest is, or if the same takes place with regard to the two sides, we may have such a state of affairs that, by sloping the ground glass, and conse quently afterwards the sensitive film, either backwards or forwards, or to one side or the other, we may get both the nearer and the more distant parts in focus.
The sloping of the ground glass is made possible by what is called the " swing-back." This arrangement is shown in the cut on the chapter on portraiture. It simply consists in causing the back portion of the camera to be movable on a hinge, instead of being rigid. There is of course a means of clamping it tightly after the desired obliquity to the axis of the lens has been given. When the back can be adjusted backwards and forwards from the perpendicular the swing is called a "vertical swing." This is the most useful adjustment, and in many of the best cameras it is the only one. In some, however, there is besides this a side-swing, whereby one side or end of the plate may be caused to be farther from the lens than the other.
All this of the differences of focal length and swinging of plates, so as to be oblique to the axis of the lens, sounds complicated when it is put in words, but we think that an illustration will make it very clear.
We illustrate the case of a subject in which one por tion is nearer the lens than another. We take the case of a sitter who sits with his face pretty well towards the camera and lens. His feet are placed somewhat forward, and are nearer the camera than his head.
It will he seen at once that if the ground glass were to have the position A B perpendicular to the as E F of the lens, the rays of light coming from the head would focus in front of it, those coming from the feet would focus behind it. If, however, we swing the ground glass so as to occupy the position C D, then both the rays from the head and the feet will come to a focus approximately upon it.
This is about the commonest use to make of the swing back in connection with portraiture. It is also used to bring both the face and chest into focus when a head and shoulders form the subject of a picture.
In landscape work there are quite as many cases in which the swing-back is as useful as in portraiture. A moment's thought will show that in almost every case the foreground is nearer the camera than is the rest of the picture. The rays from it will focus farther from the lens—that is, farther back—than will the rays from the rest of the subject. Consequently it will be an advan tage to swing the ground glass backwards.
Again, in landscape work the side-swing is frequently useful. For example, we may have on one side of our pic ture a tree or a house or what not, which is comparatively near the camera, whilst the rest of the picture is more distant. Here we may use the side-swing, swinging the back of the camera so that the side of the ground glass which receives the image of the near subject shall he farther from the lens than the other side. It is quite possible to use both swings at the same time.