Abstract: With development of display technology, not only the technical performance and cost, but also the perceived image quality has become important aspects of consideration by consumers.Visibility in sharpness differences is investigated in this paper in two experiments in which sharpness of different still images with different blurring levels is assessed by different people.Statistical analysis is made.The results show that text images are clearly more critical for judging sharpness than natural color images.In terms of the diagonal step response, the visibility threshold of sharpness differences is equal to an angular resolution of 14 arcsec for natural color images and 5 arcsec for text images.

Key words: visibility threshold, step response, sharpness, visual perception