Photoshop_User_February_2017

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SCO TT VALENTINE

Photoshop Proving Ground

how photoshop sharpens your images

Sharpening is really just contrast enhancement along an edge or boundary, so let’s see how Photoshop

sharpens your images. In terms of a digital image, an edge is just a difference in brightness and/or color

that’s spread over some distance, technically called a “discontinuity.” The distinction between an edge and

a gradient is distance relative to the size of features in an image, as well as the image itself. That is, as the

width of the edge increases, it tends to become a gradient. It’s a squishy concept but, an important one.

Note: In technical image analysis, an “edge” is a single dis-

continuity, but a “boundary” is a collection of connected

edges that form an enclosed region of the image. In order

to simplify for this article, I’ll use both terms to mean the

same thing.

In the example image on the right, the top squares have

clear edges, but as we add blur, it becomes more diffi-

cult to distinguish between the pure gray values of each

box, until the entire image becomes a continuous gradient

from white to black. The drawn curves represent the width

of each transition. If you had to choose, which set would

you say still has edges? Most photographers would say

only the top one, but consider that if you can at least make

an educated guess about the edges, there’s a chance

for some recovery by dramatically, possibly artificially,

increasing contrast to reduce the width of the edge. At

this point, we now have to consider the concept of “how

much.” How much distance and how much contrast

define an edge? This becomes an important question

when considering sharpening techniques and values—

more on that in a bit.