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MOTION PICTURE PHOTOGRAPHY
tant and is one of the principal reasons an ordinary magnifying lens is not suitable for making photographs.
It is an unfortunate fact that there are on the market today some makes of cinematographic lenses which are not fully corrected for visual and actinic focus. The writer was at one time compelled through force of necessity, to use such a lens, and it was only after making many tests to obtain a focusing scale or by focusing upon an object at a certain ratio of distance nearer the lens, that he was able to produce pictures of satisfactory sharpness with it. As it is never necessary to change the focal distance from infinity in astronomical photography, no attempt is made to correct telescopic objectives since, when actinic focus is once obtained, it is never necessary to change it.
The lens is the agent by which the light is directed to the right spot in forming the image depending upon the refraction of light. But before taking up the consideration of this important piece of apparatus for photographic work it will be necessary to explain what we mean by the "Optics of Photography" as distinguished from the optics of other sciences, such as those of the telescope and the microscope.
The chief distinctions are of two kinds: ist, in photographic optics, the lens must be capable of transmitting and bringing to a focus in the same plane oblique and axial rays of light, as shown in Fig. 20.
The principal lens or objective of the telescope will not give a sharp image of an object if removed a slight degree from the axis or perfect squareness of position in relation to the line of light. Hence, the sharpness of the image produced by the objective of the telescope is confined to a small area close to the axis. The photographic lens, on the other hand, must be so constructed that it will give a sharp image of objects in front of the center of the lens and also of those that are situated to a certain extent on each side of the center.
2d. The photographic lens must also be so constructed that it will bring to a focus at the same spot the chemical and visual rays of light. If not corrected, the lens will act as a prism and separate the light into its component parts and produce the spectral or rainbow fringe around the edges of the image.
The violet or active end of the spectrum is brought to a focus close to the lens and the red at the greatest distance. The
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