THE OPTICS OF PHOTOGRAPHY The lens is the only part of a camera which a person of average mechanical skill cannot make for himself. A good carpenter can make the camera body, dark slides, and stand, and anybody with a taste for mechanism can construct some kind of shutter. Every one taking up photography must, however, buy a lens if good work is to be done. Instructions are sometimes given in photographic books as to how a lens can be made ; but mere perusal of these working directions is sufficient to show their absurdity for the ordinary photo grapher, who has neither unlimited wealth nor a very large amount of spare time. A knowledge of the fundamental differences between various types of lenses is essential to the photographer who desires to spend his money to the best advantage. Some knowledge of the properties of a lens is necessary also, if it is to be used intelligently.
It may be thought that the early photographic workers found excellent lenses ready for use, since the telescope is a comparatively old invention ; but this was not so. The objective of a telescope is designed to bring only axial rays to a focus, and has a very small field of view. A normal angle of view for a lens used in photography is from 40 to 6o degrees ; it must bring to a focus not only axial rays, but also oblique rays. As a telescope objective was of little use, opticians had to devise a lens which would form an accurate reproduction on a plane surface of a plane object situated at right angles to the lens axis, and at the same time have a fairly large field of view. Many lenses were soon con structed which would do this ; but the amount of light they allowed to reach the sensitive plate was comparatively small, and the exposure had to be fairly long. It is only within the last twenty years that lenses have been constructed which will accurately reproduce a plane object on a plane surface, and at the same time allow sufficient light to pass to permit of the plate being fully exposed in a space of time measured in fractions of a second.
Refraction.—The formation of an image by a lens depends on that property of the material composing it which is called Refraction. When a ray of light passes from one medium to another its direction is changed, unless this direction is normal to the surface separating the media. Refraction is
the term applied to such changing of direction. The amount of change depends on the media and on the colour of the light. In Fig. 7 a ray of blue light, AB, is shown passing from a medium, a, to another medium, b, and its new direction as BC. The rays AB, BC are, however, in the same plane as the normal to the surface of separation at B. If the blue ray is replaced by a red ray, its direction is changed to a lesser extent, and will be in some such direction as BD. If PP' is the normal to the surface separating the media a, b at B, it is found that, whatever be the inclination of the ray sin
Dispersion.—If AB is a ray of white light, its constituents are refracted in varying degrees, giving rise to the familiar spectrum. This splitting up of a ray of white light into its constituents is called Dispersion. If the medium b is replaced by another medium, it is possible to select such a new medium that although the refractive index I/ is still the same as that of the medium b, the amount of dispersion is different. That is to say that, although the ray BE occupies the same position, the rays BC, BD are either closer together or farther apart. We may select such media, possessing a high refrac tive index in combination with low dispersive power, and vice versa. This is of great importance in photographic optics.