Aerial obliques may be taken at any angle, although a distinction is sometimes made between obliques of high angle and panoramic or low angle views (Fig. [146]). In addition to ordinary obliques, a very beautiful development is the stereo oblique. Both kinds of oblique photography call for special instrumental equipment and technique.
Fig. 144.—The outskirts of Arras. Low oblique showing contours.
Methods and Apparatus for Oblique Photography.—The simplest method of taking oblique pictures from a plane is to use a hand camera pointed at the desired angle. Its limitations are in the size and scale of the picture obtainable, and in the inherent limitations to the method of camera support. A step in advance of this is to mount the camera above the fuselage, on the machine gun ring or turret, in place of the gun. Considerably greater rigidity is thus obtained, and heavier cameras can be utilized, although the wind resistance is a serious factor. Excellent obliques have been made in this way, even with 50-centimeter cameras, but the scheme is impractical in military planes, because of the removal of machine gun protection.
Fig. 145.—Oblique view of Capitol and Congressional Library, Washington.
Fig. 146.—Fort Alvenslegen, near Metz.
Photo by Photographic Section A. E. F.
If the camera is fixed in the fuselage in its normal vertical position, obliques may be and have been taken by the simple expedient of banking the plane steeply. This is not to be recommended as a standard procedure, especially for taking a consecutive series of exposures.
The most satisfactory arrangements for taking obliques are two; first, to mount the camera obliquely in the plane, and second, to use a mirror or prism, in front or behind the lens of the vertically mounted camera. The first method has been employed chiefly by the French, the latter by the English, whose gravity fed cameras could not be mounted obliquely.