A large-scale psychophysical experiment was performed examining the effects of various simultaneous variations of image parameters on perceived image sharpness. The goal of this experiment was to unlock some of the rules of image sharpness perception. A paired comparison paradigm was used to compare images of different resolution, contrast, noise, and sharpening. In total, 50 people performed over 140,000 observations. The results indicate that there are several very interesting trade-offs between the various parameters of contrast, noise, resolution, and spatial sharpening. An interval scale of image sharpness was created. This scale was then used to test the results of several existing models of color and spatial vision. The ultimate goal of this experiment, along with the visual modeling is to obtain a mathematical model of perceived image quality.