Perception of image sharpness which is unrelated to actual resolution
An image with artificially increased acutanceAnother illustration, where overshoot caused by using unsharp masking to sharpen the image (bottom half) increases acutance.
In photography, acutance describes a subjective perception of sharpness that is related to the edge contrast of an image.[1][2] 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 (high acutance developers), or by optical means in printing (unsharp masking). In digital photography, onboard camera software and image postprocessing tools such as Photoshop or GIMP offer various sharpening facilities, the most widely used of which is known as "unsharp mask" because the algorithm is derived from the eponymous analog processing method.
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.
Artificially increased acutance has drawbacks. In this somewhat overdone example most viewers will also be able to see the borders separately from the line, which create two halos around the line, one dark and one shimmering bright.
^David Präkel (4 January 2010). The Visual Dictionary of Photography. AVA Publishing. pp. 19–. ISBN 978-2-940411-04-7.
^Maître, Henri (2015). "Image Quality". From Photon to Pixel. pp. 205–251. doi:10.1002/9781119238447.ch6. ISBN 9781119238447.
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