At the Edge of Acutance

Topics Covered:

  • Acutance or Edge contrast
  • Local contrast vs Global contrast

People see only what they are prepared to see – Ralph Waldo Emerson

“Yes, the eye can be cheated,” sighed Dr. Optoglass. “You’ll find visual tricks everywhere, so I don’t have to convince you of that. Sometimes the eye makes something look worse than it really is, and sometimes better than it really is.”

Was Cathy one of these visions, thought Bob.


“I won’t beat around the bush. By increasing the contrast at the edges, an image can appear sharper and more detailed. It won’t have more detail, mind you, because resolution doesn’t change. But it will appear to have more detail, and hence sharper.”

“I use the unsharp mask all the time,” ventured Paul. “It sharpens an image up real nice.”

“I’m sure it does,” replied Dr. Optoglass. “This property of the edge, by which I mean the contrast of the edge, or edge contrast, is called Acutance. So if the thickness of each line determines resolution, the contrast of the edge between two lines determines acutance. Some people like to call acutance local contrast, to separate it from global contrast.”

“Global contrast being plain old contrast, right?”

“Absolutely correct. Contrast, or global contrast, is the difference between light and dark in an image. Local contrast, or acutance or edge contrast, is the contrast of the boundary between two details. The eye is drawn to high contrast, and if this happens to be in the edges, the mind is fooled into believing the image is sharper than it really is.”

“I have a question, doc.” Paul immediately realized the word ‘doc’ wasn’t endearing to Dr. Optoglass. “I mean, doctor. When I sharpen an image I also get noise.”

“I was coming to that, hopefully without any more interruptions.”


“There’s one more thing that comes in the way of sharpness. And that is noise.”


  • Acutance is the contrast in the edges of an image.
  • Acutance is also called local contrast or edge contrast.
  • Global contrast is the overall contrast of an image.

Links for further study:

Next: What Noise Does
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