EFFECTIVE RESOLUTION OF DIGITAL FRAME IMAGES

摘要

The nominal resolution of digital aerial images does not always correspond to the effective resolution. This can be tested by edge analysis, but the resolution is just one figure about the image quality. A more detailed information is included in the modulation transfer function (MTF), showing also the image quality for lower image frequency. The MTF is quite dependent upon the used aperture. If the diameter of the aperture is reduced against the full opening, at first the image quality is improved, shown by the MTF, but if the opening of the aperture is reduced too much, the image quality is reduced. This effect is caused by the diffraction limited resolution, linear dependent upon the wave length. It shows very well that a reduction of the pixel size in the image for the same size of the CCD-array may not improve the image information contents even if the nominal number of pixels is enlarged. The limitation of the image quality caused by the optics is not a new effect for now proposed 6(mu)m pixel size, also with 7.2(mu)m and even 9(mu)m pixel size under some imaging conditions the effective resolution does not correspond to the simple nominal values. The MTF as function of radial distance from principal point usually shows a loss of quality in the image corners. These effects are obvious by a visual inspection of the images - a specification is possible by the MTF and also edge analysis over the whole image areas. The situation for the large format digital cameras is discussed. The reduced effective resolution causes a loss of details for topographic mapping. The results of mapping with images taken by the DMC, UltraCamD, UltraCamX and the analogue RC30 are shown. It confirms the result of the edge analysis and the MTF, as well as the influence of diffraction limited resolution.