Negatives taken when the plane is not flying level will be distorted (Figs. 134 and 135). Contact prints from these will not fit into a mosaic, and no mere enlargement or reduction will make them available. It is necessary with these negatives to resort to a rectifying camera. This is an enlarging camera built so that the negative and print easel may be inclined about vertical and horizontal axes, thereby purposely introducing a distortion sufficient to offset the distortion of the negative. Thus, if the bottom of the printing surface is moved away from the lens, that part of the picture will be enlarged; if moved toward the lens, reduced.
For small rectifications the common practice is to tilt the printing surface alone, a method that is practical as long as this tilting does not affect the focus so much as to require prohibitive stopping down of the lens. For great distortions, such as that inherent in the principle of the Bagley camera, it is necessary to tilt both negative and print in order to preserve an approximate focus, a given portion of the negative moving toward the lens as the corresponding portion of the print is moved away. Both schemes for rectification are shown diagrammatically in Fig. 122.