Early Work In Photography
A Text-book For Beginners

56 Early Work in Photography. is, however, grossly exaggerated. It will be seen that the lays passing through the centre of the lens pass thicitn he focus at the point F, while those passing through the of the lens focus at F, with the result that the inges formed by the two sets of rays do not coincide - images every point in the lens gives a slightly different focus we obtain on the ground-glass an infinite number of imå-es just as we did with the large pin-hole in fig 12 es, exaggeration in the diagram consists mainly in the enormous size of the lens, as compared with the size and distance of the objects In actual work the lens would, as a rule reader smaller (relatively). It will at once occur to the reader that by covering up the outer rim of the lens, and allowing only the centre to act, we shall have a greater sharpness of the image, just as we did when the pin-hole illuminated. But this image will not so strongly Fortunately both chromatic and spherical aberration can be prevented by comparatively simple means. If, glase we using one single lens ground from one piece of glass, we make it of two pieces of glass, which differ suitably in their refraction of the violet and yellow ravs we can make the two sets of rays focus at approximates the same point. In the same way, while the form of lety the have illustrated makes the marginal rays focus nearer than the central or axial rays, a different construction would cause the axial lays to focus nearer than marginal. By combining two glasses, possessing the opposite properties in the hing proportions, one will correct the other to a practicnllv sufficient extent; and with this lens of two glasses we obtain what we desire, viz., a sharp distinct image, using all the lays that can pass through a comparatively large hole In uNe are able, with a comparatively short ex- aberration. image free from chromatic and spherical This form of lens, made from two glasses (usually a crown and a flint glass) is commonly spoken of as a “single » lens and is also called an achromatic, or a landscape lens - achromatic, because its chromatism (or chromatic aberration is cured; landscape, because of its ordinary use. Even with such a lens the correction of the defects mentioned relative, and not absolute; so we introduce a “stop” or diaphragm to cut off some of the rays, and use only A portion of the centre of the lens for the central rays, while the marginal rays can only pass through the margin of the lens. Thus we make a compromise, sacrificing something