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OPTICAL SCIENCE IN CINEMATOGRAPHY 53
Rudolph's lens, however, was symmetrical and while coma causes no trouble in symmetrical lenses of low or moderate aperture it does become troublesome in lenses of high speed. No further advance of this type of lens is noted until in 1920, British patent No. 157,040 was granted to Taylor, Taylor and Hobson on a lens later described in detail by H. W. Lee in the Transactions of the Optical Society XXV, 5, Page 240-248. This lens, which has a speed of f :2.0, takes off from the Rudolph lens of 1897, improving upon it by increasing its speed four fold while maintaining good correction by departing somewhat from the symmetrical form and employing different glasses. This lens has also achieved popularity in motion picture photography.
The same principle has been followed in the design of the Bausch & Lomb Raytar lens which employs still different glass combinations in the interest of still better correction of coma. This lens has been designed to work at f : 2.3 although higher speeds are easily attainable. This is a 1928-29 product on which production has not reached sufficient volume at the time this is written to justify announcing it.
That the construction does permit greater speeds is proved by the fact that Zeiss has apparently been able to secure good image quality in a lens of f : 1.4. This is gathered from the provisional patent specifications abstracted in the Official Journal of the British Patent Office of November 21, 1928, in which a lens of this speed is described and shown to follow in the direct line of succession from the original double Gauss objective.
It would be beyond the limits set for this article to even mention all the attempts which have been made in the last ten years to produce faster lenses for motion picture photography so it has been regarded as more interesting and profitable to discuss the principles of lens construction along which useful advances have been made.
In all the efforts to improve lenses and to produce faster lenses, no fundamentally new principles have been discovered since the invention of the anastigmat. Faster lenses have been achieved with the aid of old principles of design and have been realized partly due to the fact that they are required only in comparatively short focal lengths.
With regard to future developments, there seems little hope of any further marked increase in speed. A limit is set by two considerations. In the first place we are limited by the necessity for some depth of focus. As the speed increases depth of focus decreases for conditions otherwise the same, and we soon reach a point where the depth of focus becomes too shallow for any purpose other than copying. Then, too, a limit is imposed by something like the economic law of diminishing returns. When we get into this region of ultra fast lenses exposure time is not found to decrease in proportion to the increase in speed of lens because of relatively greater losses of light within the lens itself. Light is lost at each boundary surface separating air from glass and is absorbed by the glass itself. The more complex the construction, and complexity is demanded in securing good image quality combined with high speed, the greater is the loss of light due to reflection and absorption. Unless new materials become