TELLS THE NUMBER OF PIXELS THE TRUTH? - EFFECTIVE RESOLUTION OF LARGE SIZE DIGITAL FRAME CAMERAS

摘要

The photo scale or the ground sampling distance (GSD) usually is the dominating factor for the specification of photo flights. For a comparison of cameras not only the nominal number of pixels is important, also the image quality has an influence. The effective resolution, respecting the image quality, can be determined by edge analysis. A sudden change of the brightness in the object space is causing a continuous change of the gray values in a profile across the edge. A differentiation of the gray value profile lead to the point spread function, including the information of effective resolution. DMC, UltraCamD and UltraCamX-images as well as analog aerial photos have been investigated. Of course the effective resolution is depending upon the illumination condition, expressed by the sun elevation, and the atmospheric condition. For the DMC only in one case with 20 deg sun elevation the effective GSD was 5percent larger than the nominal value; this is different for both UltraCam. The calibration reports shows lower modulation transfer functions in the image corners. Under optimal light conditions, with 60 deg sun elevation, the UltraCamD in the center has a loss 4percent of the effective GSD against the nominal value, but in the image corners a loss of 28percent exists. With 20 deg and 27 deg sun elevation an overall loss of resolution by 16percent respectively 24percent has been detected. The same effect exists for the UltraCamX, showing an effective resolution 28percent less than the nominal value over the whole image format. Similar investigations have been made for analog photos.