Early Work In Photography
A Text-book For Beginners

58 Early Work in Photography. screen is placed at B, the top and bottom of the image will be sharp, while the centre will be out of focus. The best effect is obtained by placing the plate between these two points. Fig. 17. Fortunately, as in the case of aberration, this distortion and curvature of field can be largely overcome. While a single lens with the stop in front of it gives barrel-shaped distortion, the same lens turned round, with tlie stop behind it, will give the reverse, or pin-cushion distortion, thus:— Fig. 18. and by combining two such lenses and, placing the stop between them, we form a "rectilinear” or “doublet” lens, which gives straight lines right to the margin of the picture. In the same way, while a single lens of given construction gives us curvature of field in one direction, concave towards the lens, the same lens turned round will give a curvature of field convex towards itself. Thus the rectilinear or doublet form cures two distinct defects of the single lens. Here, as in all problems of practical optics, the result is relative, rather than absolute, for we almost invariably find that in gaining a given advantage we have to put up with a corresponding disadvantage; so that the whole science of lens-making is a series of compromises. The single lens and the doublet, which is also called the