Sharpening
Processing that increases edge contrast to make an image appear crisper, without adding any real detail.
Measured in
%
In depth
Sharpening cannot recover detail that was never captured — it exaggerates the transitions that are already there, which the eye reads as sharpness. Applied too strongly it produces bright halos along edges and amplifies noise. Proper practice divides it into three stages for capture, creative emphasis and output.
Examples in practice
A file sharpened until halos appear around a roofline looks crisper on screen and worse in print.