One of the most frequently-used yet most commonly misunderstood
post-processing operations has to be "sharpening" -- technically
"unsharp masking" or USM for short. The confusion is due to a lack
of understanding of the difference between resolution and
accutance, and what the parameters (amount, radius, threshold)
actually mean. This has led to a proliferation of dumbed-down
sharpening tools that get better results than just some random
application of USM, but rarely as good as doing the job yourself
with some understanding of the process. This is also why non-SLR
digicams apply much too much sharpening in-camera.
This lesson will explain these concepts, how unsharp masking
affects them (or not), and describe two different purposes for
which unsharp masking can be used. After mastering these, you will
be able to turn down sharpening to the lowest level in your camera
and apply it yourself later, which will make your pictures visibly
better and more "professionally finished".
Sharpness, resolution, and accutance
When we perceive an image as sharp, we're actually dealing with two
separate issues: resolution and accutance (aka contrast).
Resolution means how small detail a lens/sensor can record.
Accutance means what is the lowest tone difference a lens/sensor
A lens/sensor with high resolution but low accutance will produce
images with lots of detail, but the detail won't be easily apparent
because the differences in tone that make it up will be
"flattened": images may end up with a muddy or "flat" look.
A lens/sensor with high accutance and low resolution will produce
images where the detail is very clearly visible, but there may not
be as much of it recorded as with the high-resolution,
low-accutance lens/sensor. These images can actually have more
"snap" and look sharper and more detailed than the higher-rez but
lower-contrast images produced by the previous system.
Now, resolution can't be artificially increased: detail that isn't
recorded can't be pulled out of thin air. However, accutance can.
This is what unsharp mask does: it increases the brightness
differences between adjacent areas. As long as the detail was
captured (i.e., the tonal difference was above the absolute minimum
that the system can record), it can be "brought out".
So, judicious use of unsharp mask will improve the accutance of an
image, which will make the detail that was already in it more
visible. Overuse will cause "sharpening haloes," aliasing, or other
nasty side-effects (which can't easily be undone). This is why
higher-end cameras generally apply less USM in-camera than consumer
cameras: the advanced photographer is more likely to want to
capture the best possible original where s/he can adjust the image
(using USM, among other things) for maximum effect and minimum
negative impact, whereas a "consumer" will probably not know how to
use USM properly, or will not want to post-process the images at
What does USM do?
Unsharp mask increases the accutance of an image by boosting
contrast between adjacent areas. The size of the change is
proportional to the existing differences: a completely uniform area
will not be affected at all, whereas an already high-contrast area
will be affected most. The way it does this is determined by the
parameters: amount, radius, and threshold. The amount sets how
much USM changes the contrast. The radius determines how far
USM looks from each pixel to make the adjustment. The threshold
means how much difference it looks for to make any change at all.
Each of these impacts the picture in a specific way.
Of the three variables, Radius is the pivotal one: it determines
the type of impact USM will have.
What to watch out for: haloes, noise, and aliasing
Unsharp mask has some common undesirable side-effects: it can
produce haloes (most commonly light "glows" around dark details)
and accentuate noise. Very low-radius USM can also cause aliasing
-- increase in contrast to the point that fine detail is compressed
to the level of individual pixels, which show up as jaggies.
Aliasing is an issue for pictures downsampled for the web only;
full-size pictures aren't sharp enough to start with to produce
this phenomenon before being completely ruined by haloes. (Pictures
from the Foveon sensor are a notable exception.)
So, once you see haloes, a big increase in noise, or jaggies, you
know you've overdone it.
You can see greatly exaggerated side-effects of USM in the above
picture. The (downsampled) unsharpened original is in the top
strip. Below that is as close to "optimal" sharpening as I could
manage: the picture is sharp, but artifacts are not bad enough to
be distracting (although you can see some if you look for them).
Below that is a strip that has been way oversharpened at a very low
radius, causing jaggy, harsh lines, and at the bottom is the
"classic" over-sharpening -- too big radius, too high amount,
leading to haloes. Look especially at the two badly-sharpened
areas, and see how the radius affects the look of the artifacts.
Setting the variables
The larger the radius, the more variation there's likely to be, and
therefore, the more visible the impact. So, USM with an amount of
100 and radius of 2 pixels will impact a picture much more than an
amount of 100 and a radius of 0.2 pixels. The lower the radius, the
higher the amount needed to make a visible difference, and vice
versa. So, the first thing to remember when applying USM is to
leave amount as a "loose variable." Threshold is another loose
variable: it stops USM from affecting areas with little contrast.
Boosting the threshold can be used to dampen the noise-enhancing
effect of USM.
Radius, amount... And artifacts
Unsharp mask is applied to each pixel in turn. You can think of
each dark pixel as "sucking" colour from lighter areas around it,
to the distance set by the radius, and each light pixel as
"sucking" lightness from darker areas around it. This explains the
most common and most unpleasant artifact caused by USM: the
sharpening halo. It shows up as a lighter "glow" around dark areas,
or less obviously as a dark shadow around light areas. USM with too
big a radius and too high an amount will produce such a halo.
Instead, it will accentuate noise, and will have no effect on areas
that aren't already pretty sharp. So:
USM rule 1: use as small a radius as you can...
(...and boost the amount until it looks good.)
If a picture already has high resolution, such as a photo that has
been downsampled (shrunk) to WWW size, you can get away with
extremely low radius and high-amount sharpening: I usually use 0.2
pixels and 500%, sometimes twice in a row.
However, this does not work for all pictures. For example,
full-size out-of-the-camera or out-of-the-scanner pictures don't
have as good resolution per pixel as downsampled images: when
viewed at 100%, they look blurrier and softer. Therefore, you'll
need to increase the radius until it's about half the "blur
distance" in the picture, and turn down the amount to match. For
these pictures, a radius of 0.7 pixels and an amount of 115% or so
could be a good starting point... but watch out for those haloes;
any radius starting from 0.5 pixels can create them.
So, radius and amount have a relationship: change one, and you'll
have to change the other to keep a similar impact on the picture.
USM Rule 2: set the threshold as high as you can
Threshold determines the minimum contrast difference that USM will
"grab". If set to zero, it will accentuate any differences at all.
This will have the side effect of considerably aggravating
noise/grain. Since the perception of sharpness is usually derived
from edges and areas that already have some contrast, you'll want
to boost the threshold so that most of the noise will be left
unaffected, yet the picture will be optimally sharpened. So, once
you've set the radius and amount to get the sharpening you want,
nudge up the threshold to just below the point that it cancels out
Recap: how to sharpen with USM
Here's the normal sequence for finding the proper amount of USM to
apply to a picture:
- Think of what you want to do.
- Set an Amount that's big enough for you to see the effects of
- Set a Radius that gets you the effect you want.
- Set a Threshold that minimizes the impact on noise.
- Turn down the Amount so you get the level you want with acceptable haloes or aliasing.
An extra twist: de-fogging
Unsharp mask can be used for other purposes than simply sharpening
the picture. One way can be described "de-fogging". It simulates
the optical qualities of extremely high-quality lenses, canceling
out some of the "fogginess" introduced by complex zoom lenses with
lots of lens elements and surfaces. Here, we're not trying to
sharpen small detail, but increase the contrast between large areas
of the picture. You can apply it in addition to the normal,
low-radius sharpening described above. Here are the settings:
Amount: ca 20% (very low)
Radius: ca 60 px for 5-MP image (very high)
It will have an effect of "scrubbing" the picture. It's best to do
this before curves or levels adjustments, as it will increase the
spread of the histogram, and can result in blown highlights or
clipped shadows. (See the Levels lesson for an explanation of
this.) The effect is very subtle, but it does make a difference.
When to apply USM?
It's best to apply the "sharpening" USM as the last thing before
printing or publishing on the Web. The effects of USM cannot be
fixed, and the amount of USM required depends on the picture.
However, the "de-fogging" USM is best applied as the first thing,
as otherwise you risk inadvertently clipping the highlights or the
- Pick a picture. Downsample it to web size, and use USM on it to
get it to optimum sharpness. Describe the settings you used, and
how you arrived at them.
- Pick a picture. Use USM on it to optimize the sharpness on the
full-size example. Crop a (max) 400 x 400 pixel area of it before
and after sharpening, post, and describe the settings you used and
how you arrived at them.
- Pick a picture. De-fog it with USM. Downsample the "before" and
"after" versions to 400 pixels wide, sharpen the "after" version,
and post. Describe the settings you used and how you arrived at