Acutance
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In photography, acutance is the edge contrast of an image. Acutance can be more specifically defined as the amplitude of the derivative of brightness with respect to space. Due to the nature of the human visual system, an image with higher acutance appears sharper even though an increase in acutance does not increase real resolution.
In the example image, two light gray lines were drawn on a gray background. As the transition is instantaneous, the line is as sharp as can be represented at this resolution. Acutance in the left line was artificially increased by adding a 1 pixel wide darker border on the outside of the line and a 1 pixel wide brighter border on the inside of the line. The actual sharpness of the image has been decreased because the transition takes place across 4 pixels, but the apparent sharpness is increased because of the greater acutance.
Artificially increased acutance is not without its cost. In this somewhat overdone example most viewers will also be able to see the borders separately from the line, creating two halos around the line, one dark and one shimmering bright.
A number of image processing techniques such as unsharp masking exist to increase the acutance in real images.
