CURVATURE OF FIELD.
One difficulty inseparable from the spherical form in which lenses are ground has already been dealt with, namely, spherical aberration. There is, however, another. The image of a flat object is not formed on a plane or flat surface, but on a curved one ; thus it follows that if the middle of the picture is focussed for with full aperture, the edges are out of focus, and rice versa. This fault is known as curvature of field. In the example shown in Fig. 491, A represents a lens uncorrected for curvature of field, n the diaphragm, o the object, and f f the plane of the image. It is evident that if h h be the position of the focussing screen, the only portion of the image in focus will be the central part B. Obviously, the best effect, in the circumstances, will be obtained by placing the screen midway between the extremes of focus, as shown by the dotted line, and using the smallest stop possible. Curvature of field is some times due to faulty adjustment of the lens, in which case matters may often he improved by slightly altering the degree of separation between the com binations. The best modern high-grade lenses have an almost perfectly flat field. The most satisfactory method of over coming curvature, where it is known to be present in the lens, is shown by Fig. 492.
Let Ancp represent the focussing screen. Divide this with ruled pencil lines, or mentally, into four equal divisions, using the longest side of the screen if it is rectangular. As will be evident, by comparison with Fig. -191, the figure 2, being in the centre, will fall in one extreme of the focus, while the out side edges of the plate represent the other extreme. If, then, focussing is clone on any object falling on the intermediate figures 1 and 3, the mean between the two extremes is obtained, which will give the most satisfactory result, provided the lens is well stopped down.
position for the diaphragm is explained on p. 35. In rectilinear and symmetrical lenses distortion is counteracted by placing the diaphragm between two com binations, when one kind of distortion balances the other, and both are ob literated. The barrel form of distortion is generally considered the least objection able, therefore nearly all single lenses are fitted with the diaphragm in front. The longer the focus of the lens the less the distortion that occurs ; a good quality single lens of long focus will show scarcely any appreciable amount of this defect. It is not generally known that