A Sharper Image |
I say "maligned and misunderstood" because many people only use the "sharpening" tools in image editing programs to try to correct for blurry or mis-focused original images. Worse, some manufacturers of low-end editing programs (e.g., Microsoft, with PictureIt) felt that "sharpening" was too technical a term, and so changed it to the more (mis)understandable term "focus." Aarrgghh! (Sorry, I had to get that out of my system.)
Image sharpening can do a lot of things for your pictures, but it can't fix an out-of-focus original! IT.IS.NOT.FOCUS ! We'll look a bit deeper at what image sharpening can do, and how it works, so you'll be able to apply it properly. The results can be impressive if you have a good sharpening tool, good images, and know what you're doing!
A bit of background: What is "Sharpness?"
To understand what "sharpening" routines in image editing packages do, it's helpful to understand what we mean by "sharpness" in the first place. Keep in mind though, that we need to explain the concept in terms a mindless image-editing program can understand.
With only a little thought, it's easy to see that image sharpness has a lot to do with
the edges
of things; those places in a picture where the image content
is (or should be) changing rapidly from one tone or color to
another. In a "sharp" image, these transitions happen
quickly, over a small area. In a "soft" image, transitions
between objects are blurred across a larger area.From this definition, it's easy to see that what we need to do to increase the perception of sharpness is to make transitions between one color or tone and another in the image happen more rapidly. Of course, we only want to do this where there's a fairly abrupt transition or edge already present. In computer-ese, we want the program to find places where it looks like the color or tone is changing rapidly, and make that change more rapid. In fact, this is exactly what sharpening routines in image-editing programs do.
Why stop there? Fooling Mother Nature.
In actuality, most image-sharpening techniques go a bit further than just making tonal changes more abrupt: They deliberately over-compensate a bit, and push tone and color on each side of the edge a bit beyond where they're really supposed to go. Then, a little further away from the edge itself, they let the values come back to what they should be.
It turns out that this over-compensation is just the ticket for fooling our eyes into thinking that there's more detail there than there actually is. (More than meets the eye?) Actually, it can't insert detail that doesn't exist, but it can make us more aware of what's there. The figure titled "Sharp up Close" shows an extreme magnification of an edge "sharpened" in Corel PhotoPaint, using the "unsharp masking" filter. The top half of the figure shows the original tint blocks, while the bottom half shows the impact of unsharp masking. Notice how the (grossly exaggerated) unsharp masking emphasizes the edge.
Unsharp Masking: A flash from the past.
Actually, for all our digital sophistication, all we're doing is mimicking a darkroom process that's been around for the last 50 years or so, and that's still actively practiced today. (Albeit by darkroom perfectionists with way too much time on their hands!)
The technique of "unsharp masking" in conventional photo processing involves making an underexposed, soft ("unsharp") contact-print from a negative, sandwiching this "unsharp mask" in precise registration with the original negative, and then making a print with the combination. The result is an artificial enhancement of fine detail in the print that can produce exquisite results in the hands of a skilled practitioner.
A quick run through the controls
While many of the low-end image editors include sharpening controls of one sort or another, we're going to focus on the "real thing" here; the unsharp masking operator, as found on professional packages like Adobe Photoshop and Corel PhotoPaint. If you don't have an image-editor of this level, don't despair: Most sharpening software implements the same basic techniques. By understanding how unsharp masking works, you'll be better able to recognize what the lower-end sharpening tools are doing, and use them appropriately.
The figure labeled "Unsharp Central"
shows the unsharp masking control panel from Photoshop. (Corel
PhotoPaint's is quite similar, differing only in that Corel doesn't
permit decimal values for the radius.) At top left is a small
preview window showing the effect your settings will have on
the image. You can scroll this around the picture simply by clicking
anywhere on it and dragging in the direction you want to go.
You can also zoom in or out by clicking the + or - button below
the window, although I find it most useful to work at 100%. The
"preview" check-box lets you view the effect of your
settings on the entire image. (This can take a while if your
image is large.)
The controls themselves appear at the bottom of the window: The
Amount slider dictates how strongly the effect should be applied,
while the Radius control tells the program how wide an area (in
pixels) to scan over when looking for an edge. The area specified
by the Radius control also governs how large a region will be
affected on each side of edges that are found. The Threshold
slider is more subtle: It sets a minimum "edge contrast,"
below which the unsharp masking operator won't have any effect.
In other words, minor variations in contrast will be ignored
if below this value. (The Threshold control can be very useful
when sharpening portraits: You want the hair and clothes to snap,
but don't want to emphasize every tiny blemish on the subject's
skin. By playing with the Threshold setting, you can often keep
the sharpening from affecting the subject's face, yet still work
where you really need it.)
Proper Use
I said at the outset that sharpening can't do much for an out-of-focus
original. This is because the blurring in poorly focused photos
spreads the picture information out across too wide an area to
have any hope of reconstructing it, unless you happen to work
for NASA. On the other hand, scanners and particularly printers
tend to blur images only slightly during the reproduction process,
and in a very controlled fashion. This sort of blurring is relatively
easy to compensate for with sharpening, and the results can be
impressive, indeed. The ideal amount of sharpening to use is
that which just compensates for the effects of the scanning/printing
process: Any more will produce artificial, coarse-looking images.
Learn by Doing
It takes a little practice to know how best to adjust the unsharp
masking controls, particularly since there is no single "best"
setting for all circumstances. Your settings will probably be
governed first by your printer, and then by the source image
itself. There's really no substitute for experience (with your
own setup), so I strongly recommend you commit to investing a
fair number of "wasted" sheets from your printer to
see how the controls work, and what looks best.
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This file shows a range of detail, making it a good subject for our sharpening test. |  |
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Here, the effect of sharpening is just beginning to be felt, although it probably won't be too evident in a printed reproduction. |  |
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This will probably look about right in print, although it seemed much too severe on my CRT. Notice how much more defined the veins and texture are in the leaves. |  |
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This is clearly too much! Notice how all the fine detail has been lost, and the "halos" around contrasting objects. This is particularly evident in the siding on the house in the background: Note the visible white line immediately above the shadow under each clapboard. If you see false "details" like these around the edges of your objects, you've overdone it: You probably need to reduce the amount and radius both. |  |
The first and most critical step is to decide what radius
setting to use. Too small, and there will be little or no effect.
Too large, and details will be obscured by the sharpening process
itself. (In fact, you can get some pretty wild creative effects
by deliberately using very large radius values with the unsharp
masking filter.) I generally decide on the best radius to use
by cranking the Amount control all the way up, and then adjusting
the Radius slider until I just start to see a thickening of fine
detail and "halos" around objects. Leaving the Radius
slider at that setting, I then pull the Amount control down to
something reasonable, usually around 150%.
An important note in this though: Since you're concerned about
creating a sharp print, the printed output is where you need
to look to decide how much sharpening is appropriate. Make several
copies of the same image, and try different sharpening variations
on each. Then, print them all and see which looks best. With
low-end (i.e., non-photo-quality) inkjet printers, you'll probably
find that what looks good on paper looks like way too much on
the computer screen. Part of the skill in applying sharpening
lies in knowing how to translate what you're seeing on the screen
to what will appear on paper.
As an aid to understanding the effect of the different controls,
we've cropped a section of detail from an image, and reproduced
it with here several different radius and amount settings.
Go Forth and Sharpen!
Moderate use of the sharpening tools in image-editing software
can dramatically improve your printed output, regardless of the
printer you're using. Use the tools appropriately and subtly,
and all people will notice is how great your pictures look!
