Acutance

<h2 id="tools">Tools</h2>
<p>Several image processing techniques, such as <a href="/facts/Unsharp_masking/AQfhY2hB">unsharp masking</a>, can increase the acutance in real images.
</p>

<h3>Resampling</h3>

<p><a href="/facts/Low-pass_filter/QFsUjw7h">Low-pass filtering</a> and resampling often cause <a href="/facts/Overshoot_(signal)/l450BHuI">overshoot</a>, which increases acutance, but can also reduce absolute gradient, which reduces acutance.  Filtering and resampling can also cause <a href="/facts/Clipping_(audio)/H86VUxpz">clipping</a> and <a href="/facts/Ringing_artifacts/JC2W86cf">ringing artifacts</a>. An example is <a href="/facts/Bicubic_interpolation/Fov96ol8">bicubic interpolation</a>, widely used in <a href="/facts/Image_processing/zenalNCf">image processing</a> for resizing images.
</p>
<h2 id="definition">Definition</h2>
<p>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.
</p><p>Summing the slope Gn of the curve at N points within W gives the acutance value A,
</p><p>
  
    
      
        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}}

</p><p>More generally, the acutance at a point in an image is related to the <a href="/facts/Image_gradient/ImvsRE52">image gradient</a>, the <a href="/facts/Spatial_gradient/2cmoCezd">gradient</a> of the density (or intensity) at that point, a vector quantity:
</p><p>
  
    
      
        A
        =
        ∇
        D
      
    
    {\displaystyle A=\nabla D}

</p><p>Several <a href="/facts/Edge_detection/9VplpIcS">edge detection</a> algorithms exist, based on the gradient norm or its components.
</p>
<h2 id="sharpness">Sharpness</h2>
<p>Perceived sharpness is a combination of both <a href="/facts/Optical_resolution/TcLkPYFw">resolution</a> 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. 
</p><p>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).
</p><p>Coarse <a href="/facts/Film_grain/UDdv279z">grain</a> or <a href="/facts/Image_noise/zwhrU8lc">noise</a> can, like sharpening filters, increase acutance, hence increasing the perception of sharpness, even though they degrade the <a href="/facts/Signal-to-noise_ratio/qohClhyG">signal-to-noise ratio</a>.
</p><p>The term <i>critical sharpness</i> is sometimes heard (by analogy with <a href="/facts/Critical_focus/kEH9hUw2">critical focus</a>) for "obtaining maximal optical resolution", as limited by the <a href="/facts/Sensor/RN8SXbjs">sensor</a>/<a href="/facts/Film/wM3b9Kk3">film</a> and <a href="/facts/Lens/ejlH6n5c">lens</a>, and in practice means minimizing <a href="/facts/Camera_shake/DrBT5QVS">camera shake</a> – using a <a href="/facts/Tripod_(photography)/hBAUSSdG">tripod</a> or alternative support, <a href="/facts/Mirror_lock-up/4HSVYXrY">mirror lock-up</a>, a <a href="/facts/Cable_release/5uRW43Lh">cable release</a> or timer, <a href="/facts/Image_stabilization/DrBT5QVS">image stabilizing</a> lenses – and <a href="/facts/Optimal_aperture/8PV11GQm">optimal aperture</a> 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).
</p>
<h2 id="see-also">See also</h2>
<ul><li><a href="/facts/Contrast_(vision)/zD9F91Je">Contrast (vision)</a></li>
<li><a href="/facts/Cornsweet_illusion/Mz2RCKJA">Cornsweet illusion</a></li>
<li><a href="/facts/Edge_enhancement/o7CIqhCt">Edge enhancement</a></li>
<li><a href="/facts/Hyperacuity/RWEstLLd">Hyperacuity</a></li>
<li><a href="/facts/Mach_bands/nubDv69P">Mach bands</a></li>
<li><a href="/facts/Ringing_artifact/JC2W86cf">Ringing artifact</a></li></ul>

<h2 id="further-reading">Further reading</h2>
<ul><li><i>The Focal Encyclopedia of Photography</i>, Focal Press, 1956, Ed. Frederick Purves</li></ul>
<h2 id="external-links">External links</h2>
<ul><li><a href="http://www.cambridgeincolour.com/tutorials/sharpness.htm">Tutorials: Sharpness</a>, at Cambridge in Colour</li>
<li><a href="http://www.luminous-landscape.com/tutorials/sharp.shtml">Lens Sharpness: The Never-Ending Quest</a>, at <a href="http://www.luminous-landscape.com/">The Luminous Landscape</a></li></ul>

<h2 id="references">References</h2>

<ol>
<li id="fn:1"><p>David Präkel (4 January 2010). The Visual Dictionary of Photography. AVA Publishing. pp. 19–. ISBN 978-2-940411-04-7. <a href="978-2-940411-04-7" target="_blank">978-2-940411-04-7</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li>
<li id="fn:2"><p>Maître, Henri (2015). "Image Quality". From Photon to Pixel. pp. 205–251. doi:10.1002/9781119238447.ch6. ISBN 9781119238447. <a href="9781119238447" target="_blank">9781119238447</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li>
</ol>

Acutance open-in-new

Acutance