International projectionist (Oct 1931-Sept 1933)

WHY ANASTIGMATIC LENSES? David Levinson and Alvin A. Hill THE widespread introduction of "corrected" or anastigmatic lenses in projection work, prompted by changing conditions in the motion picture field and the constant desire to improve the quality of projection, is a matter of comparatively recent record. Projection lenses generally in use before the advent of anastigmatic lenses have in principle been of the Petzval type used in portrait work, or of the shortfocus type, "which amounts essentially to a telescope objective with an achromatic condenser mounted a short distance ahead of the film, which functions simultaneously as part of the illuminating system and part of the projection objective. Both of these types are characterized by the very sharp central definition and a very limited portion of the field which can be called flat." An anastigmatic lens is one that has been fully corrected for spherical aberration, astigmatism, and chromatic aberration. Spherical Aberration Spherical aberration arises from the nature of the curve used in "uncorrected" lenses. These lenses have spherical curves — that is, they are ground as part of a sphere. The consequence is that a section of a sphere, or an ordinary projection lens, not having curvature enough towards the center, has an infinite number of foci at different distances. There is a difference of the refractive power of different portions of the same lens, the marginal portions of the lens having an excess of refractive power as compared with the central portions which excess of refraction increases with the distance from the center. Astigmatism, a form of spherical aber Connecting D. C. Generators in Parallel (Continued from preceding page) coupled, provided, of course, their voltages are equal and provided their resistances of the series field-coils are inversely proportional to the current capacities of the two generators — that is, if one generator produces twice as much current as the other, its series-coil should have one-half resistance. It is further necessary that the two generators should agree in their action, so that a given increase in load will produce the same effect upon their voltages. If they are not in agreement, they may be adjusted by slightly increasing the resistance of the series coil of that generator which tends to take too large a share of the load. ration, is created when rays from an object passing obliquely through a lens converge through two focal planes instead of coming to a single point. The images thus formed do not focus with clean-cut lines on the screen. There also is a decided lack of contrast, and the projected image is more nearly a monotone in gray than a true black-and-white. "It is an unfortunate provision of nature," writes one scientist, "that a single lens applied to the task of forming an image gives us an image about as far from the quality we want as could well be and still have it recognizable as an image. The ideal image is the true projection of the object spaces onto a plane, such a projection as might be constructed by drawing single lines from every point in the object space through a pinhole and continuing them until they intersect the desired plane of projection. "The images formed by a lens differs from this ideal in many respects. In the case of the pinhole, the image is equally sharp no matter at what distance from it lies the plane of projection, for it is assumed to be so small that only a single ray of light from any one object can pass through it. The lens, however, is of finite size and many rays from any one object are received by the lens. For perfect performance all these rays should be reunited by the lens in another point in the desired plane of projection (the focal plane of the lens). "It happens otherwise, however, as is illustrated in Figure 1. Here there is represented an object point 0 lying in the margin of the field. Instead of the lens forming a point image of 0, it forms as the nearest approach to it an elliptical spot of light at 0'. If we explore the cone of light in the neighborhood of 0', we will find that it nowhere comes to a point focus. At the place marked t in the diagram the light seems to be concentrated in a short line as indicated and, at another place, such as 5, it again seems to be concentrated into another line at right angles to the line at t. t is the focus for the meridian of the line marked t, and 5 is the focus for the corresponding meridian of the lens. "The phenomenon of the representa tion by a lens of object point 0 as a pair of perpendicular lines is called astigmatism. The distance from 0' to the center of the distance between t and 5 is the curvature of field for this angle. The astigmatic difference (distance between t and 5), and the curvature of the field will vary from point to point over the field depending on the angle of the field of view. The focus of all points t and 5 is a pair of curved surfaces which constitutes the image of the object plane. These surfaces are indicated by the dotted curves connecting t and s in the figure with the center of the image." Correction Methods Spherical aberration and astigmatism may be overcome by the use of diaphrams with bright light, which cut off all but the central rays, but in this case distinctness is obtained at the expense of brightness. Again, spherical aberration may be reduced by using several very flat lenses instead of one thick lens. The most satisfactory correction, however, can be obtained with anastigmatic lenses ground on a system of computed curves that hold the accuracy of the lens surface curvature to within a few millionths of an inch. These lenses have similar refracting power throughout, which, under existing projection systems, is necessary to give the most evenly projected picture. Chromatic Aberration The dispersive (spreading), power of lenses, which are considered to be made up of any number of tiny prisms which bend light, is responsible for chromatic aberration. Chromatic aberration causes images to be reproduced on a screen with fringes around them, particularly around titles when the background is decidedly dark in contrast to lighter central features. The serious defect of chromatic aberration, as the projectionist already knows, is most easily and quite effectively remedied by combining a convergent lens of crown glass with a divergent lens of flint, to form a convergent achromatic lens; or, if the crown glass is di Figure 1 [14]