Contents 1 Tools 1.1 Resampling 2 Definition 3 Sharpness 4 See also 5 References 6 External links

Tools Several image processing techniques, such as unsharp masking, can increase the acutance in real images. Unprocessed, slight unsharp masking, then strong unsharp masking. Resampling Low-pass filtering and resampling affect acutance. Low-pass filtering and resampling often cause overshoot, which increases acutance, but can also reduce absolute gradient, which reduces acutance. Filtering and resampling can also cause clipping and ringing artifacts. An example is bicubic interpolation, widely used in image processing for resizing images.

Definition One definition of acutance is determined by imaging a sharp "knife-edge", producing an S-shaped distribution over a width W between maximum density D1 and minimum density D2 – steeper transitions yield higher acutance. Summing the slope Gn of the curve at N points within W gives the acutance value A, A = ( D 1 − D 2 ) 1 N ∑ n = 1 N G n 2 {\displaystyle A=\left(D_{1}-D_{2}\right){\frac {1}{N}}\sum _{n=1}^{N}G_{n}^{2}} More generally, the acutance at a point in an image is the gradient of the density (or intensity) at that point, a vector quantity: A = ∇D. Thus the acutance of an image is a vector field.

Sharpness Perceived sharpness is a combination of both resolution and acutance: it is thus a 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 critical sharpness 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 The Focal Encyclopedia of Photography, Focal Press, 1956, Ed. Frederick Purves

External links Tutorials: Sharpness, at Cambridge in Colour Understanding Sharpness, at The Luminous Landscape Lens Sharpness: The Never-Ending Quest, at The Luminous Landscape Retrieved from "https://en.wikipedia.org/w/index.php?title=Acutance&oldid=825725365" Categories: Image processingScience of photography