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As it is therefore to be seen that the Petzval objective possesses neither complete freedom from astigmatism, nor a perfectly flat field of sufficient angular extent to permit of perfect definition over the entire extent of the projected motion picture image, it appears that in order to achieve the utmost in sharpness in cinematographic projection some of the modern types of highly corrected objectives could be utilized to advantage in this class of work. Projection objectives of the photographic anastigmat type have been used n England and on the continent for some years past with great success, and I need not therefore apologize for directing the attention of the Society, and the industry at large, to the improved results which may be secured by the use of projection objectives of modern design.
The number of difl^erent types of highly corrected or so-called anastigmat photographic objectives now on the market is very large, but only a few types are eminently suitable for projection purposes, the principal requirements of a projection objective, in addition to the abolition of the previously mentioned aberrations, being:
(i) Large eff"ective aperture.
(2) Minimum of {glass-air) reflecting surfaces.
(3) Minimum of cemented surfaces.
(4) Compactness of construction, /. e., short distance between the
front and rear components.
When these requirements and also the corrections for flatness of field and freedom from astigmatism are taken into consideration, the selection narrows down considerably, and of the many modern photographic objectives available I direct your attention to only two specific types, these being the Cooke objective series 11/4.5^ the Zeiss I C Tessar/ 3.5.
For the purposes of cinematographic projection both the Cooke Series II and the Zeiss I C Tessar can be made with eff"ective apertures of/ 4.5 or even/ 3.5, and beyond an eff'ective aperture of/ 3.5 it seems extremely inadvisable to go, because with further increase in the effective aperture the aberrations inherent in all objectives can no longer be suppressed to the degree at which they are invisible in the projected results.
The influence of residual aberration upon the screen images produced by objectives of extremely large eff^ective apertures may be readily demonstrated by projecting a test slide, or film, containing printing or any other fine contrasty pattern. It will then be found that the blackness of the letters, or pattern, is much more pronounced when projection is accomplished with an objective of moderate aperture than is the case when one of extreme aperture is employed, because the residual comatic and astigmatic errors which are inseparable from objectives of large aperture tend to gray the projected image, thus robbing it of a certain degree of snappiness or proper contrast.
The Cooke lens has six glass-air surfaces, as has also the Tessar, both being identical with the Petzval objective in this respect. In view of the light loss by reflection at glass-air surfaces an objective
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