PHOTOGRAPHIC INSTRIIMENTS.—Without the camera and its lens, the majority of photographic processes would be practically useless. The photographic camera obscura—dark chamber—with its image-forming lens, may be likened to the human eye, whose retina finds its mimicry in the sensitive photographic plate upon which the image falls, and is impressed. The eye is a perfect optical instrument, whose lens and iris, or diaphragm, so adjust themselves in focus, and to the varying conditions of light, as to transmit faultless images of outer objects to the retina. The camera-lens, modelled on the same plan, transmits what are termed perfectly achromatized images to the prepared photographic plate. In lieu of an iris expanded and contracted by muscles, the camera-lens carries metallic diaphragms, and has a mechanical arrangement, by which it may be focussed for near or distant objects.
lens is of primary importance in photography ; it becomes necessary, therefore, to point out the conditions which fit the lens for its photographic functions. It will simplify matters to employ the term "objective" in place of lens, since all useful photographic lenses are combinations of two or more lenses of different foci. The first condition is to have the objective perfectly achromatized, i. e. the visual and chemical foci coincident, in other words, the chemical rays which impress the image upon the plate, and the rays which form the visible image upon the screen, should have their focus at the same point. The objective should be also free from spherical aberration ; that is to say, the image transmitted by the combined lenses should neither be curved nor distorted.
Should the double-convex lens a (Fig. 1078), and the double-concave lens b, be made of crown glass, chromatic aberration will be represented by the points of focus c d. The parallel rays of white light incident upon the lens a will be split up into seven coloured rays, each having a different degree of refrangibility, and a different focus. The red rays, having a small index of refraction, will have their focus at c, while the extreme violet rays, having a greater index of refraction, will have their focus at d, and the distance between c and d will represent the chromatic aberration of the combined lenses. Taking advantage of the difference in the
dispersive power of flint- and crown-glass, if the lens a is made of crown-glass, whose index of refraction is 1.519, and dispersive power 0.036, and the lens b of flint-glass, whose index of refraction is and dispersive power 0.0393, and if the focal length of the crown-glass is 4i in., and that of the flint-glass 73 in., they will form a lens of 10 in. focus, and will refract white light to a single focus free from colour. By this means, is obtained an achromatized combination suitable for photographic purposes. Spherical aberration may be completely corrected by the forms of the combined lenses. Thus, by combining a miniscus lens (Fig. 1079) a with a double convex lens b, the aberration produced by the lenses when used singly will disappear.
The result of having the objectives corrected is that they throw a sharply defined image upon the screen of the camera, and produce a sharply defined image upon the photographic plate. Suitable objectives must therefore be what are termed "achromatic combinations," and are made by uniting two or more lenses of flint- and crown-glass of different refracting powers.
Objectives supplied by well-known makers fulfil, as a rule, all the conditions necessary for successful photography. They distribute illumination equally over the plate, and are free from flare and distortion.
The objective commonly used in portraiture is made up of four lenses, two of flint-, and two of crown-glass. Seeing that the majority of portraits are taken indoors, with a comparatively feeble light, the portrait-objective must be used with a wide aperture, so as to admit the maximum of light. Even with the best portrait-objective, when no dia phragm is employed, the imago is apt to lack clearness and definition on its outer edge.