View source for Acutance

{{Short description|Perception of image sharpness which is unrelated to actual resolution}}
[[File:Accutance.svg|frame|An image with artificially increased acutance]]<!--Don't scale the image, it will greatly reduce the effect-->
[[File:Usm-unsharp-mask.png|thumb|Another illustration, where [[Overshoot (signal)|overshoot]] caused by using [[unsharp masking]] to sharpen the image (bottom half) increases acutance.]]

In [[photography]], '''acutance''' describes a subjective perception of [[visual acuity]] that is related to the edge [[contrast (vision)|contrast]] of an [[image]].<ref name="Präkel2010">{{cite book|author=David Präkel|title=The Visual Dictionary of Photography|url=https://books.google.com/books?id=f7X5vYbUd0sC&pg=PA19|date=4 January 2010|publisher=AVA Publishing|isbn=978-2-940411-04-7|pages=19–}}</ref><ref name="Maître2015">{{cite book|last1=Maître|first1=Henri|title=From Photon to Pixel|chapter=Image Quality|year=2015|pages=205–251|doi=10.1002/9781119238447.ch6|isbn=9781119238447}}</ref> Acutance is related to the [[vector magnitude|magnitude]] of the [[image gradient|gradient]] of [[brightness]]. 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 [[Image resolution|resolution]].

Historically, acutance was enhanced chemically during development of a [[Negative (photography)|negative]] (high acutance developers), or by optical means in printing ([[unsharp masking]]). In [[digital photography]], onboard camera software and image postprocessing tools such as [[Adobe Photoshop|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.

== Tools ==
Several image processing techniques, such as [[unsharp masking]], can increase the acutance in real images.

[[File:Accutance example.png|thumb|Unprocessed, slight unsharp masking, then strong unsharp masking.]]<!-- Again, don't scale the image. -->

=== Resampling ===
[[File:Gibbs phenomenon 10.png|thumb|[[Low-pass filter]]ing and [[Resampling (audio)|resampling]] affect acutance.]]
[[Low-pass filter]]ing and resampling often cause [[overshoot (signal)|overshoot]], which increases acutance, but can also reduce absolute gradient, which reduces acutance.  Filtering and resampling can also cause [[clipping (audio)|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 D<sub>1</sub> and minimum density D<sub>2</sub> – steeper transitions yield higher acutance.

Summing the slope G<sub>n</sub> of the curve at N points within W gives the acutance value A,

<math display=block>A = \left(D_1 - D_2\right) \frac{1}{N}\sum_{n=1}^{N} G_n^2</math>

More generally, the acutance at a point in an image is related to the [[image gradient]], the [[spatial gradient|gradient]] of the density (or intensity) at that point, a vector quantity:

<math display=block>A = \nabla D</math>

Several [[edge detection]] algorithms exist, based on the gradient norm or its components.

== Sharpness ==<!-- Redirected from [[Sharpness (visual)]] -->
Perceived sharpness is a combination of both [[Optical resolution|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 [[film grain|grain]] or [[image noise|noise]] can, like sharpening filters, increase acutance, hence increasing the perception of sharpness, even though they degrade the [[signal-to-noise ratio]].

The term ''{{visible anchor|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 (photography)|tripod]] or alternative support, [[mirror lock-up]], a [[cable release]] or timer, [[image stabilization|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]]
* [[Hyperacuity]]
* [[Mach bands]]
* [[Ringing artifact]]

== References ==
{{reflist}}
== Further reading ==
* ''The Focal Encyclopedia of Photography'', Focal Press, 1956, Ed. Frederick Purves

== External links ==
* [http://www.cambridgeincolour.com/tutorials/sharpness.htm Tutorials: Sharpness], at Cambridge in Colour
* [http://www.luminous-landscape.com/tutorials/sharp.shtml Lens Sharpness: The Never-Ending Quest], at [http://www.luminous-landscape.com/ The Luminous Landscape]

[[Category:Image processing]]
[[Category:Science of photography]]