Abstract
The estimation of the best combination of lens and film for aerial reconnaissance photography is not a simple problem, owing to the number of variables, such as focal length and aperture of the lens, the size and speed of the film, forward motion and speed of the aircraft which are involved. For quantitative considerations it is necessary to adopt a measure of the detail-revealing capacity of the negatives, and for this purpose the resolving power was chosen, since it was not only obviously related to the capacity of the negative to reveal detail, but also capable of justification as a possible means of estimating the performance of the lens in optical image formation. In order that the resolving power figures may correspond as closely as possible to practical performance, careful choice of test-object was necessary, for experiment showed that different test-objects gave different results. A test-object consisting of two rectangles separated by their width, with a difference in density of 0.2 between them and the background, was adopted. Apparatus with such test-objects at the focus of a collimator of long focal length was set up, and with it measurements were made of the photographic resolving power all over the field, and in a set of planes embracing the focus, for a representative collection of air camera lenses. The results showed that the photographic resolving power improved as the lenses were stopped down from their maximum apertures until very small apertures were reached, and that as a general rule the resolving power diminished towards the outer parts of the field. The variation with aperture was accounted for, in general terms, by theoretical considerations, and certain regularities in the change of resolving power with angular distance from the optical axis were also explained. A figure for the performance of any lens was obtained by averaging the resolving power over the field of the lens, having regard to the greater area covered at the larger angles. It was found that the performance of any of the lenses could be represented with very fair accuracy by the formula
R = [207/fG]1/2 [F.No/tan2 θ]0.3,
where R is the mean resolving power in lines per mm. over the field up to an angular distance theta from the axis, f is the focal length of the lens in inches, and F.No its relative aperture. G is the granularity of the film in density-microns at a density of 1.0.
This formula has no substantial theoretical basis, although it can be accounted for to some extent. It has proved useful, for it assists in the choice of the focal length, aperture and angle of view most appropriate for any given purpose in aerial reconnaissance photography. It may also be used, at least until the progress of lens design renders it inaccurate in form, for estimating the merit of a given lens design by comparison of its performance with that given by the formula.