American cinematographer (Oct 1933)

October 1933 • American Cinematographer 21 1 Engineering Applied to Lenses Upper left Fig. 3 and upper right Fig. 4. Both aperatures 5-2. Lower center Fig. 2, Lens System for Photography from aircraft. Aperature 5-4.5. William Taylor, O. B. E.* scientific, that is, based upon the conscious application of principles of physics and mathematics, but in its main in- spirations it is an art, based on tradition and upon habit gained by experience. There are few first-rate artist-scientists of this kind in the world, men who, having a clear statement of each problem, perceive by instinct, nourished by experience, the main direction in which the solution must be sought. These pursue it by laborious mathematical work, sometimes for months, securing a first approximation, determining its de- viation, and correcting their course accordingly, again by a mixture of art and science. And of these few men it may be truthfully said that the best are Englishmen. For during the last twenty years nearly Fig. 6. Large Lens for Photographing star fields. every single important advance in this field, nearly every step in providing faster lenses and better definition, has been made in England. A perfect photographic lens would— (a) receive a large amount of light from the object, (b) condense to a mathematical point all the light which it received from each mathematical point of the object, (c) arrange the image points in correct order and relative positions so as to render the image a true visual replica of the object, (d) set all points of the image in one plane (for photographs must generally be flat), (e) illuminate all points of the image correspond- ingly with those of the object. It is physically impossible to devise a lens which has all these virtues simultaneously. Consider a solid sphere of any transparent homogeneous material such as glass. That would act as a lens, and have this one supreme virtue, that it would present the same aspect to all points of the object alike and would both ar- range the points of the image in correct relative positions along any given radial line, and illuminate each strictly in proportion to that of the corresponding point of the object. But unfortunately a solid spherical lens would not form a flat image of a flat object or a distant scene, but one curved spherically, nor would any point of the image be sharply defined; and this for two reasons:— (1 ) No single lens, of whatever shape, can focus, simultaneously, light of different wavelengths. (2) The wave fronts after passing such a lens would no longer be spherical but deformed, and, un- less they be spherical, they cannot converge to mathematical points. These two defects are respectively termed chromatic and spherical aberration. To cure them necessitates the use of at least two glasses of different compositions such that the relative velocities of light of different wavelengths are sub- stantially different in the several glasses, the forms of the lenses being adapted to these properties of the glasses. Lenses formed in this way, of two glasses, commonly suffice for the objectives of telescopes, whose angle of view rarely extends to 10° and whose apertures are generally about one-twelfth the focal length (f-12). But with the best photographic lenses the angle of view sometimes reaches 100°. Apertures have now reached f-1.3, and such lenses must be substantially free from dis- tortion and image curvature, chromatic and spherical aberra- tion, astigmatism and coma (these being varieties of spherical aberration affecting light passing obliquely to the (Continued on Page 240)