Improving field characteristics of photographic objectives has always been the concern of the lens designer. Progress has followed the introduction of glasses with high index and low dispersion. These new glasses have been used in the collective lenses in such a manner as to increase the index difference between collective and dispersive elements. This has the effect of basically moving both tangential and sagittal focal points closer to the focal plane throughout the zonal portions of the field, improving Petzval sum, and increasing covering power. Variations of zonal corrections and covering power with these basically improved characteristics are discussed.
The high index glasses disclosed in U. S. Reissue 21,175 (Morey) and in the more recent patents by DePaolis and Eberlin have been used in a number of lens systems. Comparative examples are shown to illustrate the extent to which Petzval sum is improved and/or covering power is increased with the use of these glasses.
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