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1949 OBJECTIVE LENSES 97
to 20 lines per millimeter on the axis and only 5 to 8 lines per millimeter at the corner of the 16-mm frame while at least one recent massproduced//!^ lens can resolve 60 lines per millimeter and 45 lines per millimeter at the center and edge, respectively, of the same field on the same high-speed emulsion. Extreme lens speeds are valuable in those applications where object brightness is low as in cine fluorography, where exposures are extremely short as in ultrahigh-speed photography, and in those situations where short exposure times are necessary at locations of low light level where the use of lamps is objectionable. They may also be useful in projection where exceptionally high levels of screen illumination must be obtained or where a given high screen illumination must be maintained with an economy of light-source power or with light sources of relatively low brightness.
An optical disadvantage of these extreme-aperture objectives, aside from lack of high resolution, is the extremely shallow depth of focus. In order to realize the full resolving power of an //0.8 objective capable of separating 40 lines per millimete-r the emulsion must be positioned within 0.016 mm (0.0006 inch) of the image plane. Clearly the taking or projecting equipment intended for use with an objective of such aperture must be properly designed and fabricated. It is also important to recognize the low light-transmission factor in refracting objectives, where, notwithstanding the use of low reflection coatings, there still will be considerable light loss at the 10 or 12 glass-air surfaces in addition to significant loss due to long glass paths. For those applications where a considerable amount of the light is at the violet end of the spectrum there may be a disproportionately large loss due to the use of extremely dense flint elements in the design.
Testing of extreme-aperture taking lenses is best performed by photographic means, preferably utilizing the subject contrast and type of emulsion with which the objective is to be employed. Visual tests on an optical bench are convenient, although* evaluation is difficult because of the high magnifications which accompany the use of large numerical-aperture microscope objectives and partly because the observation is subjective. It is essential that the numerical aperture of the microscope objective used on the optical bench be at least as great as that of the objective under test. In testing an objective of //I, //0. 85, or//0.6 aperture, the numerical aperture of the microscope objective must be not less than 0.5, 0.59, or 0.83, respectively, otherwise the observed image will be that formed by the objective when stopped down to the aperture corresponding to the numerical