Amplitude spectra of natural images

Several studies have suggested that the amplitude spectra of photographs of natural scenes are remarkably similar and have the form: amplitude varies; is directly proportional to spatial frequency-1.0. This is, of course, a straight line with slope of -1.0 when plotted on double logarithmic coordinates. We have examined the amplitude spectra of 135 digitized photographs of natural scenes and have found that relatively few images conform exactly to the suggestion. About 25% of the images in our sample have spectra which show significant curvature when plotted on log-log coordinates. The best-fitting regression lines have slopes that range from -0.8 to -1.5; the average slope is -1.2, rather steeper than previously suggested.