712 



HANDBOOK OF PHOTOGRAPHY 



fuselage is preferable to one with side by side arrangement, as in the latter case the 

 pilot has difficulty in seeing downward on the far side of the ship and usually has to 

 move back and forth from one seat to the other. Stability is another essential charac- 

 teristic of the mapping airplane, as instability results in nonuniformity of overlap, 

 badly tilted pictures, and furthermore keeps the pilot so busy in attempting to 

 maintain an even keel that his efforts toward precision navigation are seriously 

 impaired. The mapping airplane, in addition to the usual instruments, needs to have 

 an exceptionally good compass, which is free from lag and undue oscillation. A 

 sensitive-type altimeter is also important because the ordinary type of airplane altim- 

 eter has so much lag that the elevation 

 can change appreciably before the alti- 

 meter is affected. Another instrument 

 which is important is a strut thermometer 

 giving the temperature of the outside air. 

 This is essential in making corrections to 

 the altimeter reading. 



Camera. — A great variety of camera 

 equipment is employed in the various 

 phases of aerial photography and aerial 

 mapping. One particular company 

 engaged in this type of work utilizes 15 

 different kinds of cameras, each for a 

 different purpose. 



For oblique photography, the camera 

 question is comparatively simple. 

 Acceptable obliques have been secured by 

 homemade cameras and modified stand- 

 ard cameras, although a regular aerial 

 camera is desirable. Factors to be taken 

 into consideration in selecting a camera 

 for oblique photography are 



1. Focal length. The focal length 

 'should be as long as practical. A com- 

 promise must be effected between the 

 long focal length which is desirable and 

 weight and mobility. The camera must be light enough so that a man can easily 

 handle it, point it, and move quickly with the camera in his hands from one subject 

 to another. The most satisfactory all-round focal length for oblique photography is 

 12 in. This lens is generally used on a 7- by 9-in. photograph. 



2. Speed. Lenses for oblique photography should have a speed of //4.5 or faster. 



3. Filters. The lens should be equipped with a bayonet type of filter, so that 

 filters may be switched in the air as different conditions of haze, visibility, and light are 

 encountered. 



4. Exposure. Oblique photographs are generally exposed at M50 sec. or faster. 

 The camera should be equipped with a couple of solid grips for holding and pointing 



it, with a trigger which is convenient to be operated while holding the camera by the 

 grips. It should have a single-winding action, which changes the film .and resets the 

 shutter in one operation. An ordinary negative glass finder with peep sight is quite 

 satisfactory-. 



The camera requirements for vertical photography are much more complex. In 

 common use today are focal lengths ranging from 2 to 24 in. Cameras vary in con- 

 struction from conventional single-lens instruments to nine-lens instruments. Various 



Fig. 6. — Nine-lens camera for making 

 aerial photographs, mounted in floor of 

 plane. Note oxygen arrangement. 



