We use Optical Character Recognition (OCR) during our scanning and processing workflow to make the content of each page searchable. You can view the automatically generated text below as well as copy and paste individual pieces of text to quote in your own work.
Text recognition is never 100% accurate. Many parts of the scanned page may not be reflected in the OCR text output, including: images, page layout, certain fonts or handwriting.
136
HANDBOOK OF PROJECTION FOR
or light source be a pin point, a lens will not focus the light beam to a pin point. A lens is presumed to form an image, and an image always has area. The matter should be viewed thus: Rays emanate from each pin point of the object, or light source. (When projecting a light source, which is exactly what the condenser does, the light source becomes an "object," within the meaning of that term as here used.) The rays from the particular pin point, and all other pin points of the object or light source, are picked up by a considerable area of the lens, or perhaps by its entire area, and the rays from each particular pin point of the object are refocused to a corresponding, though perhaps a magnified or reduced point in the image. That is what is meant by "focus." It is illustrated in Fig. 30.
A little study will enable the student projectionist to understand why he is unable to focus the condenser ray to a pin point. In the case of the condenser ray, the condenser is receiving rays from the entire area of the floor of the carbon crater, and is refocusing them to an image of the crater. By reason of the fact that the crater does not set square with the condenser (the lower part being farther away from the lens than the upper part, and the further fact that the uncorrected lenses used for condensers set up spherical aberration) the actual image of the crater will be focused over a considerable distance in the condenser ray. Only a certain portion of the crater will therefore be in sharp focus at any one point in the condenser beam. Since the crater itself has considerable area, the image will have area, hence the beam cannot be focused to a point.
PROJECTION^EENS.— A projection lens is in fact a combination of four lenses, two of which are located at the end of the lens tube nearest the film. These two lenses are separated by a spacing ring by some manufacturers, but not by others, and the combination is commonly referred to as the "back factor" of the projection lens. The two other lenses located at the opposite end of the lens tube are cemented into direct contact with each other by means of Canadian Balsam, and are commonly referred to as the "front factor" of the projection lens. The front factor, usually being
Figure 34,