››PHOT OSHOP PROVIN G GROUND› ›kelbyone.com059Fortunately, digital images are
made up of pixels that come with
a defined size, so that takes care
of how to measure distance (just
not how much distance matters).
And each pixel has a single value
of color and brightness. If we’re
only looking at two pixels, we
can easily see any differences,
and the boundary is pretty clear.
Adding more pixels makes it a bit
more challenging.
Now let’s imagine one black
and one white square as if they
were single pixels—really, really
big pixels. Since pixels them-
selves are a fixed size, we can’t
do much about changing the boundary width unless we
add more pixels to the edge width. But we can choose
brighter or darker gray values by increasing contrast.
Now pretend we’re looking at an edge between black
and white, and the width of the edge is three pixels with
50% gray in the middle. I’ve added Curves adjustments
below to demonstrate the changes.
Making the edge pixels brighter causes the edge to
appear to shift toward the black, while making them darker
moves the edge the other way. You can test this quickly by
moving further back from your screen, and squinting a bit.
If you do this, tell anyone watching that you’re having a
staring contest. Ask them to choose the winner.
In any case, we’re manipulating the rate of change in
the edge. Instead of being a linear, smooth transition from
black to white, we’ve created a curve that makes the transi-
tion width appear smaller by adjusting the contrast (which
is relative). This really has to do with perception and how
we see these transitions. Creating too much contrast leads
to artifacts like halos, and sometimes to jagged edges. Let’s
see how Photoshop actually performs sharpening.FlatDarkenedBrightenedContrast