CHAPTER 9 HOW SOFTWARE CHANGES PIXELS BY THE NUMBERS^147
To determine which pixels it will change, the
digital darkroom compares neighboring
pixels to see if the tone of one exceeds the
color of another by a preset threshold.
(You can, with some photo editing pro-
grams, set the threshold to fine-tune the
degree of sharpening applied.)3
If the contrast between the
pixels doesn’t meet the
threshold, the pixels aren’t
changed.4
If the contrast between adjacent pixels
exceeds the threshold, the lighter pixel is
lightened by a preset amount and the
darker pixel is darkened.5
Another number the soft-
ware considers is called
theradius. If the radius is
set to 0, only pixels being
compared are changed, as
shown here. The radius
can also be set to a larger
number so that more
of the surrounding
pixels are also light-
ened and darkened.
However, setting the radius
number too large results in
an unnatural look, with
halos around the sharp-
ened edges of the subjects
in your photos.6
When Unsharp Is Sharp
Photo editing programs have more than one way to sharpen an image. Commands such as SharpenandMore Sharp don’t require a
lot of explanation. The function described here is called Unsharp Mask, a term that seems to be going in the wrong direction. It gets its
name from the fact that areas with significant changes in contrast look sharp while areas without big changes in contrast look smooth and
less defined and are therefore effectively masked from the sharpening filter.