In photography, the term acutance describes a subjective perception of sharpness that is related to the edge contrast of an image. Acutance is related to 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. Historically, acutance was enhanced chemically during development of a negative, 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 one-pixel-wide darker border on the outside of the line and a one-pixel-wide brighter border on the inside of the line. The actual sharpness of the image is unchanged, but the apparent sharpness is increased because of the greater acutance. In this somewhat overdone example most viewers will also be able to see the borders separately from the line, several image processing techniques, such as unsharp masking, can increase the acutance in real images. Low-pass filtering and resampling often cause overshoot, which increases acutance, but can also reduce absolute gradient, filtering and resampling can also cause clipping and ringing artifacts. An example is bicubic interpolation, widely used in processing for resizing images. Thus the acutance of an image is a vector field, coarse grain or noise can, like sharpening filters, increase acutance, hence increasing the perception of sharpness, even though they degrade the signal-to-noise ratio
Another illustration, where overshoot caused by using unsharp masking to sharpen the image (bottom half) increases acutance.
Unprocessed, slight unsharp masking, then strong unsharp masking.