International projectionist (Jan-Dec 1947)

BLUE FIGURE 4 positive crown and negative flint lens to produce the basic type of achromatic doublet. We have seen in Fig. 2 that a ray of light, upon passage through a prism, is bent, or deviated, in the direction of the base. This is essentially the fundamental reason for the action of lenses of all kinds. The curved surfaces act like an assembly of an infinite number of small prisms, deviating each ray striking the surfaces sufficiently to bring it to a reunion, real or virtual, with the other rays forming the image. A positive lens will converge parallel rays to a real focus; while a negative lens will diverge parallel rays, making them act as if they came from a point, the virtual focus. From what was said previously concerning dispersion, it is apparent that any simple lens cannot have one definite, fixed focal point for all light. Since the light-bending power, or refractivity, of glass is greater for blue than for the red, the blue light will focus at a point nearer the lens than the red. This situation is illustrated in Fig. 3. This is the simplest and most readily grasped type of chromatic aberration, and usually the first corrected. In practice, this longitudinal chromatic aberration will mean that there is no one focal point on the axis but several, depending on the color of the light used. A photograph made with a simple positive lens would show a large shift from visual focus to photographic, even with panchromatic negative material. The "chemical focus" of the old-time photographers was of this nature. Axial Chromatism Correction A perfect lens cannot be made, and even in the best lenses there remains a very small residual of this aberration, so that when a color-blind emulsion responding only to the blue is used, a shift towards the lens is usually necessary— the so-called "chemical focus." This effect is familiar also to those who have used infra-red sensitive emulsions in their cameras: for best results, it is usually necessary to rack the lens out a trifle. A further result of this irresolution of focal points is the situation shown in Fig. 3, where at the blue focus the red rays create a red disc, and at the red focus the blue rays create a blue halo. A point object could hardly be photo graphed as a point under these conditions. This axial chromatism is not difficult to correct and, as noted before, is given high priority. The secret lies in the relation of dispersion to deviation. Consider for a moment a simple positive lens as shown in Fig. 3. The marginal rays have been deviated toward a focus, and at the same time because of the dispersion of the glass, the red and blue rays are aimed at different points on the axis. Now, everything would be perfect if there existed an optical material with a given amount of dispersion and no refractive power, for then correction could lens. The lens component effecting this achromatism is negative, as shown in Fig. 4, and must have higher refractivity and dispersion than its positive mate. This combination, then, will bring light of any two colors to a common focus on the axis. The other colors will focus at points practically identical with the chosen colors. Thus this lens would give a color-free star image on the axis. Other Chromatic Aberration The other type of chromatic aberration is a bit more difficult to understand. It is somewhat more complicated both to explain and to show in a drawing. Some of us may recall mention in our reading concerning optics of certain things called "cardinal points," "ideal planes," etc. These points and planes are convenient ways of describing the properties of lens systems and are indispensible to the lens designer. Briefly, these cardinal points can be looked on as points on the lens axis at which the refractive BACK FOCUS be effected with a plane parallel sheet of this wonderful material. Actually, the only practical material for this task is a glass which has a fortuitous relationship of refractivity to dispersion such that the dispersion will effectively cancel that of the positive lens, while the refractivity is insufficient to cancel completely the convergence of the positive FIGURE 6 WW FIGURE 5 powers of the lenses or lens system are concentrated. The cardinal points and planes are exceedingly useful because they simplify computations by replacing a complex, almost unmanageable system by points at which all the refraction can be considered to occur, or more graphically, by thin lenses whose laws are simple and easy to handle. Irrespective of the distance from ihe NODAL POINTS WHITE < INTERNATIONAL PROJECTIONIST • January 1947 11