A laboratory simulation of high altitude photography on three aero emulsions at scales from 1/100,000 to 1/800,000
Harvey, David I
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The employment of miniature camera systems in high altitude photography has not been considered practical until very recently. In the past, the quality of lenses and films has been such that long focal length lenses and consequently large formats were necessary te record the desired ground detail. With recent advances in high speed optics and emulsion making, the potentialities of miniature camera systems have greatly increased. It is now possible to construct systems of this type having resolutian capabilities four or five times as great as conventional large format systems. The purpose of this work was to simulate certain aspects of high altitude photography, in order to study: 1) the potential of a 35 mm laboratory camera, 2) the photographic quality and amount of information that could be obtained on three different aero emulsions, 3) the effect of atmospheric haze on 2, and 4) to determine whether more information could be extracted by viewing a projected image of a negative or a print of that negative. An appraisal of the factors influencing the picture quality of aerial photographs was made. Such problems as aircraft motions, camera design and installation, atmospheric effects, and the brightness and contrast of scene detail was investigated. It was concluded that with proper installation and design of the camera system that three main factors influence the quality of high altitude photography of a given target scene. These factors are the quality of the camera lens, the ability of the film to record fine detail, and the amount of atmospheric haze. For this study a high acuity 35 mm laboratory camera was constructed. The lens used with this camera system was a 50 mm f/2.8 Schneider "Xenotar". To provide an extremely fine and wide range of focus this lens was adapted to an interferometer table on which a Leica Focaslide assembly bad been mounted. Leiea If and Leica IIIc camera bodies were used with the system to provide a wide range of shutter speeds. The lens aperture was held constant at f/5.6 throughout the experiment, an aperture at which the lens was essentially diffraction limited. Three Kodak films were used in this study; Micro-File, and two aerial emulsions, SO-1213 and Plus-X Aerecon. MicroFile, a non-aerial film, was included in order to show what could be done with a high resolution film, although in practice its speed is so low as to require extremely fast optical systems. SO-1213, a new experimental film, is one of the finest grained emulsions yet developed for aerial use. Plus-X Aerecon, just recently put into production, has a speed comparable ta Super-XX, but with much better image quality. Atmospheric haze was simulated by placing a diffusing disc inside the lens just behind the iris diaphragm. A circular hole, whose size corresponded with the diaphragm aperture at f/5.6, was bored out of the center of this disc. By opening the diaphragm aperture beyond f/5.6, the diffusing disc added a uniform "layer" of scattered light to the image, thus effectively simulating atmospheric haze. Three aperture settings were used in this work, f/5.6, f/4, and f/2.8. The following haze factors were obtained. at these apertures: 0, 0.14, and 0.36. Haze factor is defined here as the ratio of the haze brightness to the scene high-light brightness. The target scene consisted of four high quality positive transparencies of Boston, Mass. at a scale of 1/20,000. A standard Air Force resolution target was inserted in a corner of one transparency. The target was illuminated by a light panel which had been specially constructed to provide a very even source of diffuse daylight illumination. The brightness range of the illuminated aerial scene was approximately 40:1. Photos were taken at distances such that the resultant scales on the 35 mm negatives were 1/100,000, 1/200,000, 1/400,000, and 1/800,000. The simulated altitudes were 5, 10, 20, and 40 miles, respectively. Thirty-six negatives were obtained using the above system, one for each film, at each scale, and at each haze setting. Care was taken to insure optimum focus. Using a high quality enlarging system, prints of each negative were made. All prints were enlarged to the same scale. The print enlargements for negatives at the fourscales, 1/100,000, 1/200,000, 1/400,000, and 1/800,000, was 3X, 6X, 12X, and 24X, respectively. Three methods were used in evaluating the results; 1) a microscopic examination of the negatives for resolution, 2) a microscopic examination of the prints for recognition and detection of certain scene detail, and 3) a qualitative examination of a greatly enlarged projected image of the negatives. The average resolution in lines per mm for the three films tested using haze factors of 0, 0.14, and 0.36, respectively, was found to be: For Micro-File: 174, 158, and 148; For SO-1213: 107, 93, and 80; and for Plus-X Aerecon: 57, 51, and 45. Microscopic investigation of the prints of the three films at the various scales for detection and recognition of scene detail revealed the following: At a scale of 1/100,000 with Micro-File and SO-1213 objects the size of automobiles could be detected. Aircraft and railroad cars could be easily recognized. The only difference between the two was that detail in the SO-1213 prints was less sharp. At this scale with Plus-X Aerecon automobiles were just barely visible, recognition of aircraft was difficult, and the individual cars of trains could not be seen. At 1/200,000 automobiles were barely detectable on the MicroFile prints. Trains and aircraft were still visible, but classification of aircraft type was questionable. Cars were no longer detectable on the SO-1213 prints and aircraft was barely detectable. With Plus-X Aereeon neither cars nor trains were discernible. Aircraft was just barely detectable. Even small streets could not be seen. At 1/400,000 with Micro-File large aircraft could just be detected and only relatively large buildings could be recognized. Street patterns were still plainly visible. On the SO-1213 prints aircraft and the patterns of small streets were lest. Ships and large buildings could still be seen. At this scale (enlargement of 12 diameters) the prints of Plus-X Aerecon appeared very grainy. Only relatively large structures such as large streets and buildings could be recognized. At 1/800,000 large buildings, bridges, ships, street patterns, wharfs and docks were still distinguishable on the Micro-File prints. With SO-1213 only large buildings and the rough outline of harbor facilities could be recognized. Street patterns were only recognizable in high contrast regions of the scene. The Plus-X Aerecon prints at this scale revealed only very large roadways and buildings. Outlines of harbor facilities were very rough. Under the conditions of this study, relatively small loss of picture quality was caused by introduction of haze. On the Micro-File prints virtually no loss in quality was noticed even with a .36 haze factor. On the SO-1213 prints a slight loss could be detected at .36 haze at the smaller scales in the low contrast regions of the scene. The prints most affected by haze were those of Plus-X Aerecon, the film having the lowest contrast of the three tested, and this was only noticeable at 36% haze. It must be concluded that the majority of detail sizes in the negatives were such that the compression of brightness range by haze did not reduce the detail contrasts below the visual threshold. The high photographic contrasts of these emulsions is therefore valuable. Investigation of projected images of the negatives with a projector of moderate quality revealed that the same amount of information could be obtained as on the prints. However, this method presented the problem of having to view the image from an angle plus the problem of having to interpret a negative image. Results obtained using this method were purely qualitative and its importance is only one of interest. The final results indicate that high acuity miniature camera systems, in conjunction with high resolution emulsiens, offer information gathering capacity comparable to larger, heavier cameras using standard relatively coarse-grained emulsions.
Author Misnumbered pages, but all are present. Labled 55,57,55,58. Thesis (M.A.)--Boston University
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