colorexperts

When you see a sharp edge in an image, it's because the difference between dark and light—that is, contrast—is higher along that edge than in other areas. When you apply a sharpening filter in a program such as Photoshop, you boost contrast only along edges, creating the illusion of heightened sharpness. But it isn't just edge contrast that affects whether you perceive an image to be sharp. Every adjustment that affects overall contrast contributes to what looks like sharpness. You can apply contrast to the entire image, to specific tonal ranges within the image, and to specific areas.