combination of the captured resolution, which cannot be changed in processing, and of acutance, which can be so changed. Properly, perceived sharpness is the steepness of transitions (slope), which is change in output value divided by change in position – hence it is maximized for large changes in output value (as in sharpening filters) and small changes in position (high resolution). 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. The term is sometimes heard (by analogy with critical focus) for "obtaining maximal optical resolution", as limited by the sensor/film and lens, and in practice means minimizing camera shake – using a tripod or alternative support, mirror lock-up, a cable release or timer, image stabilizing lenses – and optimal aperture for the lens and scene, usually 2–3 stops down from wide-open (more for deeper scenes: balances off diffraction blur with defocus blur or lens limits at wide-open). See also Contrast (vision) Cornsweet illusion Edge enhancement Mach bands Ringing artifact References Further reading The Focal Encyclopedia of Photography, Focal Press, 1956, Ed. Frederick Purves External links Tutorials: Sharpness, at Cambridge in Colour Lens Sharpness: The Never-Ending Quest, at The Luminous Landscape Image processing Science of photography