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The subjectively assessed quality of photographs taken with a 48 in lens has been compared with the MTF of the lens measured in the same image planes. The image quality and associated MTF were varied by the through focus technique, using a range of image planes on both sides of the optimum. The photographs have been examined to ascertain the change in MTF, at the various levels, resulting in a just detectable difference in image quality, and whether a "reduced" MTF could be used which would sensitively indicate the changes. The results will be used to provide MTF production tolerances for lenses used in aerial photography. The subject matter of the photography was vehicles as photographed from a vertical range of 40,000 ft, giving a scale of 1:10,000. Actual photography from an aircraft introduces uncontrollable factors such as vibration and atmospheric turbulence which degrade the image, so the situation was simulated using two stages of ground photography. The first produced a diapositive of vehicle models, using a high quality 35 mm lens and film which caused minimal reduction in the spatial frequency content of the original scene. This diapositive was photographed with the test lens, on to the film most often used in aerial reconnaissance operations, in conjunction with a collimator to provide the infinite conjugate condition and final correct image scale. EROS 200 equipment, employing crossed slits and a rotating grating was used to measure the MTF and the techniques employed to ensure that the image and MTF were recorded in the same plane are described. The spectral bandwidth of the illuminant was restricted to ensure that the same spectral conditions applied during the photography and MTF measurement. This enhanced the performance of the lens and broadened the scope of the experiment. Results indicate that for a combination of this lens with FP3 Aerial Film there are two useful indicators of image quality. (1) The area between the lens MTF (scaled to a maximum input modulation of 0.2) and the film threshold curve. (2) The cut-off spatial frequency. The two indicators are linked geometrically.
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