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Discussion
Dr. Kellner : Answering first a personal question by Dr. Story, whether lenses with as short a focal length as 3^ inches and a free diameter of 1^ inches are practical for motion picture projection and whether they should have been used in his tests, I may state that such lenses are really beyond the limit of commercial manufacturing possibilities at the present status of the art. The field of angle of a 3^-inch lens taken diagonally across the aperture plate amounts to 18 degrees, which cannot be covered with that lens aperture without noticeable curvature of field and other aberrations at the margin of the field.
It may be of interest to know that we have designed and made two years ago, 3^ inches f :2 projection lens in which we obtained a perfect flatness of field by the use of an parabolic surface on one of the components. Although this result represents a remarkable progress in the making of projection lenses such lenses are at present not to be considered as commercial possibilities.
Lenses with considerably greater equivalent focal length can be made without difficulty to cover the size of the aperture plate with an aperture ratio f :2. Equivalent focal lengths of about 5 inches with an aperture ratio f :2 have been made without the use of an aspheric surface, which cover the aperture plate as well as lenses of lower aperture.
Dr. Story's paper corroborates — as it seems to me — experimentally some of the statements which I made in my Cleveland paper, "On the Function of the Condenser in the Projection Apparatus," to which I have to refer for details.
An investigation of this nature will apply to the problem of the motion picture engineer only, when the comparisons are made at a location where the aperture plate may be placed in practice, i. e., somewhere in the cone between condenser and projection lens, where there is a section with an area of even light distribution large enough to cover the aperture plate.
Unless a spherically corrected condenser is used, such an area of even light distribution will be surrounded by a ring of greater intensity which is caused by the spherical aberration and which cannot be utilized for projection because it would cause uneven illumination on the screen.
However, I again like to draw attention to the fact that even if a spherically corrected condenser were used and the section through the cone at the aperture plate were evenly illuminated, considerable loss of light occurs by reason of the diaphragm action of the aperture plate. In case of the uncorrected condenser we have a combination of losses by spherical aberration and by diaphragm action.
Dr. Story : If I understand Dr. Kellner correctly, he means that even if you had a condenser having no spherical aberration — you
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