American cinematographer (Nov 1930)

November, 1930 AMERICAN CINEMATOGRAPHER Eleven FOCUSING DISTANCES FIG.3 FIG. 3. The D G curve of the secondary spectrum in lenses in use in pre-panchromatic times. It is to be especially noted that the types of lenses which are best adapted to reduce the Chromatic aberration to a mini- mum, are the worst offenders in respect of curvature of field and astigmatism corrections, and it is literally true that the aperture and the quality of the color correction are both limited by the necessity of producing a flat field. Since this article is intended to briefly discuss the correction of the chromatic aberrations for lenses as developed for cine- matographic work, we shall disregard all other aberrations and construe that the optical system under consideration is exempt of all errors but those introduced through the impossibility for modern high speed photographic lenses to focus accurately in the same plane all the various color images. This deficiency is, as stated above, entirely independent from conception of design or workmanship and is inherent with the physical properties of glass. The index of refraction of every single piece of glass, varies for all the lights of different color and thus, if a single piece of glass is ground and polished in the shape of converging lens, its power of convergence varies for the different colored rays. Since the convergence of the blue rays is greater than that of the red rays, the single lens will have a shorter focal length for the former than for the latter. If the differences between the refractions for the various colored rays, which phenomenon is called “dispersion,” were proportional to the refraction for all glasses, there would be no remedy for chromatic aberration. However, the ratio of refraction to dispersion varies for glasses of different composition and density and if two glasses are chosen, one having a high refraction for low dispersion and the other a low refraction for the same dispersion and from them two lenses are made, one positive or convergent and the other negative or divergent which combined form a positive whole, it is quite evident that such combination can be designed in which the chromatic errors of one lens are neutralized by the similar but opposite errors of the other resulting in a combination reasonably free from chromatic aberration. Such lenses are called ACHROMATIC. It must be borne in mind, however, that “dispersion” is not a finite entity, which can be fully expressed with numerical mathematical exactness and since it only expresses an infinite number of differences between the refractions for the various colored rays it is confined to express a mean between them, or between two chosen colors. Dispersion for a certain glass is usually determined by sub- tracting the refracting index of the colored light corresponding to the “F” line (Blue-Green) of the spectrum from that of the “C” (Red line) the wave lengths of which are respectively 486.15 and 656.30 micro-millimeters. When this mean dispersion is considered in the calculation of achromatic lenses, only these two colors will be accurately brought to the same focus while the others will fail to do so and cause therefore the existence of a residual aberration. This phenomenon is known by opticians as the “Secondary Spectrum” effect. Some reduction of the secondary spectrum effect is indeed obtained by bringing to the same focus three instead of two colored light rays, but this entails an increase in the number of glass surfaces and a reduction in the speed of the lens which is appropriate for some special work such as the trichrome reproduction of paintings and colored objects but which would be in opposition with the speed requisite of objectives for cinematographic work. The only optical instrument in which opticians have suc- ceeded in correcting the secondary spectrum is the “apochro- matic” microscope objective and for this the correction has been made by substituting the crown glass which would ordinarily be used by a cristallyne mineral, Fluorite. This practice cannot, however, be extended to photographic objectives simply because Fluorite cannot be obtained in suf- ficiently large pieces of optical clear quality. For example, to make a 3" lens working at a maximum aperture of F 2., (Continued on Page 22) FOCUSING DISTANCES FIG. 4 FIG. 4. Secondary spectrum curve illustrating the nearest possible compromise for coincident visual and photographic foci.