Movie Makers (Jan-Dec 1936)

How lenses were refined 540 R. FAWN MITCHELL IN the discussion, An optical excursion, in November Movie Makers, I outlined the basic elementary factors from which the modern, high speed, highly corrected lens has been evolved. Historically, like automobile design, where a motor and other equipment were attached to the horse buggy, lens design consisted of adding one more element to correct this, and another to correct that. The "traditional" trend of lens design followed the path of correcting each element to get larger apertures. It was not uncommon for some of these early lenses — very poorly corrected, by modern standards — to have ten or more glass elements. Obviously this could not go on indefinitely, so that it was only a matter of time before a new conception was developed. To H. Dennis Taylor belongs the credit for perceiving that the mamy lens aberrations could be cancelled automatically by separating the individual glasses instead of cementing them together. It is interesting to glimpse behind the scenes for a minute and to see just how such a fundamental idea was reasoned out. Mr. Taylor hit on the basic conception that really only two elements were needed — one a collective, or positive, lens and the other a dispersive, or negative, lens. Reasoning from the Law of Conservation of Energy, as well as drawing on his years of optical research, he saw that the aberrations — or lack of correction — would be eliminated if the two lenses were of equal, or approximately equal, power. Two lenses of equal power — one positive and one negative — placed together obviously would cancel each other, but by separating the two elements, the combination could form a real image. The greater the separation, the greater the power of the combination. This was an idea of inherently fundamental importance and was the first radical step taken away from traditional lens design. The principal benefit of this new arrangement was that curvature of field, which had been so troublesome up to this time, easily could be corrected, irrespective of the amount of the separation, while allowing an almost unlimited choice of refractive indices. To correct distortion, which is dependent upon the position of the diaphragm, Mr. Taylor hit upon a further happy expedient and modified the fundamental two element ideal by dividing the collective (positive) lens and placing one part of the positive element on one side and the other part of the element on the other side of the negative element. This gave additional help in controlling chromatic aberration (remember Additional facts about lens design and practicality Schemes of modern cine lenses that are here outlined the large choice of refractive indices of glass that could be used), and for the first time made it possible to achromatize a lens without first achro matizing each separate element. For the reasons outlined above, it was also possible to correct spherical aberration and coma by suitable choice of curves for the various elements. Astigmatism was corrected by the location of the stop, it being further from the collective elements than from the dispersive element, as is shown in Fig. 1. Thus, by balancing the curvatures properly, refractive indices, dispersion and spacing between glasses, all the important lens aberrations have been corrected, by using only three elements. Furthermore, they have been corrected so well that this lens will focus detail as small as can be registered on any negative emulsion ; in fact, this construction is used in the very expensive process lenses used in the highest grades of engraving work. The triplet design has three other advantages. First, no cement is used, so that possible cement deterioration is eliminated. Second, with only three glasses, there is less loss, due to internal reflections, and also the elements employed are comparatively thin so that light is not absorbed by excessive thickness of glass. The "brilliancy" of the image obtained with this type of lens is well known. Third, unique skill has been involved in making use of the air spaces between the glasses; they actually act as lenses. For instance, if the front glass is moved forward, leaving the other two as is, the focus is changed. In fact, a "front focusing" model of the one inch //3.5 lens was put out several years ago. A more interesting property, made use of in some models designed for studio portraiture, is the controllable diffusion. If the middle element is moved closer to the rear glass, leaving the other dimensions alone, spherical aberration is introduced. The greater the displacement of the center element, the greater the diffusion. In practice, the triplet principle is modified in fast, telephoto and special lenses. The functions of any one of the three fundamental glasses can be split up between two or more glasses. This is necessary to carry through the corrections of the triplet still further than is needed for an aperture of //3.5 or so. Fig. 2 illustrates diagrammatically how a negative element, strong in proportion to the positive element (or elements), reduces the physical length of a lens while retaining the same focal length. Thus, in Fig. 2, the ordinary optical construction would require a lens in which the front element would be a distance of AC from the film, whereas the telephoto construction shortens the lens to a length BC. The advantage of such compactness is obvious. Also note [Continued on page 547]