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AMATEUR MOVIE MAKING 131
follow the development of the lens briefly by considering these faults and their remedies.
Chromatic Aberration. — This is a fault of the lens which causes it to separate the colors of white light just as we saw in the case of the circular prism. As objects are seen in various colors, it is evident that we must bring all colors to a focus at one point. Otherwise we get a diffused or "soft focus" picture. By combining a positive lens (one which is thicker in the center than at the edges) with a negative (one which is thinner at the center than at the edges) we accomplish our result. The positive lens brings light rays together, and the negative spreads them. This change of direction has a magnitude depending upon the refractive power of the glass, so by using glass of two kinds we can bend the converging rays -outward just enough to compensate for the color dispersion and still have the rays meet in a common point or "focus."
Spherical Aberration. — The achromatic lens which we made by combining a positive and a negative lens, bends the rays more sharply at the edges than at the central portion. This again gives us a diffused image which is independent of color. By changing the shape of the surface from a true spherical curve and by adding a negative lens we can correct this.
Curvature of Field. — The lens tends to give a saucer-shaped field. By further altering the curvature we can flatten this field so that a picture upon a flat film will be rendered sharp throughout its area.
Linear Distortion. — To overcome some of these faults we exclude the marginal rays by use of a shield which has a round hole pierced in it. This is the diaphragm. In modern lenses this diaphragm is adjustable and is known as the iris diaphragm. This makes straight lines bend in one direction or another depending upon its position before or behind the lens. By placing a lens on both sides of the diaphragm, we correct this and secure the rapid rectilinear lens, which has four times the speed and one-half the focal length of the corresponding simple lens.
Rapid Rectilinear Lens. — When the rapid rectilinear lens was developed, it was hailed as the ultimate in lens perfection. It placed in the hands of the photog