Sharpening is necessary for all digital images, whether from a digital camera or scan of a film image

Sharpening creates an illusion of sharpness by increasing the contrast of edges

Sharpening will not correct an out-of-focus image or correct for a low quality lens

In-camera sharpening at one of several levels of intensity of JPEG images is an option, but it is best to turn it off if post-processing is to be employed

Sharpening needs to be implemented after all other processing, especially resizing

The principal tool used for sharpening is the seemingly unaptly named "Unsharp Mask" (USM)

USM is controlled by three parameters, and some practice is required to use it effectively