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 photographer, 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 60 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 constructed 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 AB, the ratio sin<ABP, is constant. This quantity is called the sin<CBP.

Refraction Photography 12

Fig. 7.

Refractive Index of the media a, b for the blue light under consideration. It is usually denoted ub. Similarly, the ratio sin<ABP sin<DBPor ur is constant, and is the Refractive Index for the red light under consideration. Now, if we are to be able to compare different glasses with respect to their refracting power, it is obvious that they must be examined as to their effect on light of a definite colour. In optics it is customary to use the light from a sodium flame which is approximately monochromatic. The refractive index for light of this colour is the refractive index for any media, and is denoted u. The refracted ray corresponding to this value is shown at BE.

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 u 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 refractive index in combination with low dispersive power, and vice versa. This is of great importance in photographic optics.

Lenses

A lens is a portion of a refracting medium bounded by spherical surfaces. The refracting medium is usually glass : but other materials, such as quartz, etc., are sometimes used. Various lenses are shown in Fig. 8, and a modern photographic lens consists of various combinations of these simple lenses of various kinds of glass. The older lenses were made with Crown and Flint glass, the crown glass having the lower refractive index and the lower dispersive power. For some time it was believed that unless glass could be made possessing high refractive power, but only low dispersive power or vice versa, no lens could be constructed which would possess a flat field and be anastigmatic.

Accordingly, the glass-makers set to work and produced such glasses, these being put on the market in 1886. The new glasses are known as Anomalous Glasses, and are indicated in the diagrams throughout this book by horizontal shading thus :

Lenses Photography 13

Crown glass is indicated.

Lenses Photography 14

and flint-glass.

Lenses Photography 15

Light will always be considered to travel from left to right.

Equivalent Lens

The modern complex lens can be replaced for purposes of calculation by a single thin lens 1, Fig. 8, which is called the Equivalent Lens. This thin lens is, however, ideal, and possesses no aberrations. It may here be mentioned that the image formed by the lens 1 suffers from various defects known as aberrations; but our thin lens is considered as being able to form a plane reproduction of an extended plane object at right angles to its axis. If, therefore, the properties of the lens 1 for narrow axial pencils of light are known, this knowledge can be applied to all pencils, whether oblique centric or oblique excentric.

Equivalent Lens Photography 16

Fig. 8.