AERIAL PHOTOGRAPHY 713 



manufacturers supply aerial cameras either with or without interchangeable maga- 

 zines and the cameras are classified as precision or nonprecision, depending upon 

 whether they are suitable for merely taking vertical aerial photographs or whether 

 they are calibrated instruments supplying pictures which may be made into contour 

 maps. 



A whole chapter could easily be devoted to a discussion of aerial cameras suitable 

 for vertical photography. The following characteristics are common to all such aerial 

 cameras : 



1. The camera must be suitable for suspending in a vertical mount, taking a 

 picture through the floor of the airplane. 



2. The mount must be of the gimbal type or equivalent, permitting the leveling 

 of the camera in flight. 



3. The camera must have level bubbles. 



4. Provision must be made for crabbing of the camera, to take care of conditions 

 when the airplane is moving over the ground in a different direction from which it is 

 headed owing to cross winds. 



5. The camera must be of fixed infinity focus. 



6. The shutter should have a speed range of from 3^o to Hso sec. The shutter 

 should have a diaphragm which may be stopped down to as much as //20. 



7. The camera must have a release trigger which can be operated from the back 

 of the camera, as the lens is generally not accessible. 



8. The winding mechanism must simultaneously change the film and cock the 

 shutter. 



9. The capacity of the film chamber should be at least 100 exposures and pref- 

 erably 250. 



10. The camera must be equipped with a view finder suitable for measuring pro- 

 gressive overlap of the pictures, or else an auxiliary instrument for this purpose must 

 be available. 



11. It is desirable to have a camera in which focal lengths are interchangeable. 



12. The focal plane of the camera must hold the film exactly flat at the instant of 

 exposure either by placing it against a glass plate or by air pressure or suction. 



13. In the focal plane of the camera collimation marks must be provided which 

 show in each photograph enabling the recovery of the principal point of the picture. 



14. The camera must be made free from static discharge on the film. 



15. All camera controls must be large and easily handled so that the operator, even 

 though wearing heavy gloves, can adjust it quicklJ^ 



For oblique photography, either a between-the-lens or a focal-plane shutter is 

 suitable. For vertical photography, when pictures are to be used for compilation of 

 precise maps, only a between-the-lens type of shutter should be employed. 



With the above general specifications for the aerial camera, a choice must 

 be made between narrow-angle and wide-angle single-lens instruments; or multilens 

 instruments. 



When large-scale detailed pictures are desired, such as would be most suitable for 

 city-planning work, a long-focus narrow-angle lens should be used. This permits 

 the securing of pictures of the same scale at a higher altitude and minimizes distortions, 

 which will be discussed later. At the other extreme of single-lens camera utility, we 

 find a wide-angle lens of around 6-in. focal length covering a 9- by 9-in. picture. Such 

 a camera would be used in small-scale mapping in order to cut down the mmiber of 

 pictures and the amount of flying and is particularly valuable in mapping high ground, 

 such as the Rocky Mountain section, where it is not possible for the airplane to ascend 

 high enough to utilize longer focal lengths of cameras. For some uses, such as military 

 and very low coast reconnaisance surveys, multiple-lens cameras are used. Multiple- 



