Feature: The Secret of the Unsharp Mask Tool

Do you think your digital camera takes pictures sharp enough to give you a paper cut? Your focus may seem pretty good--until you look closely at some pictures on a big 17-inch monitor. That's when you may see that the focus was ever-so-slightly off. Or your images might be a tad blurry because your camera's focus is always a bit "soft." Whatever the reason, fret not. You can improve your photos with a sharpening tool like Unsharp Mask, which is found in most image editing programs.

Despite the Name, It Sharpens

The Unsharp Mask tool has an odd name, but it's usually your best bet for sharpening a digital picture. Most programs have several sharpening tools. The ordinary ones--with names like Sharpen or Sharpen More--tend to tweak the wrong parts of a picture and make it look grainy, like an old 35mm photo.

The Unsharp Mask tool enhances the apparent sharpness of an image by increasing the contrast in the color of adjacent pixels. But instead of adjusting that contrast willy-nilly throughout your picture--as some sharpening filters do--Unsharp Mask increases the contrast only around natural edges in the picture (regions of dramatic color change).

Sounds great, right? Well, a lot of people skip this tool and use the less-effective Sharpen tool instead. Why? Because the options in the Unsharp Mask dialog box look intimidating. It seems better to just click once than have to fuss with numbers and do some experimenting.

This week we'll end your Unsharp Mask phobia once and for all, so you can sharpen your pictures without worry. Open the Unsharp Mask tool now and take a look at your options. In Paint Shop Pro, you'd do that by choosing Effects, Sharpen, Unsharp Mask.

Strength

Depending upon the program you use, the Unsharp Mask options might have different names. Strength is sometimes called Amount, for instance. This variable refers to how much sharpening you are applying. In other words, it controls the contrast change along the edges in your photo, making dark colors darker and light colors lighter. As a general rule, 100 to 150 percent is the most useful range for this variable. Start with 100 and see if you like the effect. You can always try it again with a higher number. But be careful about increasing it too much; you can end up with a noisy photo with harsh edges.

Radius

This next option lets you specify how far from an edge--measured in pixels--the effect should go. I typically set my radius to 1 for a small image and 2 for larger multimegapixel images. Like Strength, setting this variable too high can result in harsh edges with lighter pixels around all the edges.

Clipping

The last of the Unsharp Mask options is called either Clipping or Threshold, depending upon your image editing program. This variable controls how much contrast must already be in the image before the tool will apply any sharpening. The lower the number you enter, the more sharpening you'll get.

But watch out: If your image is grainy or has digital noise (random pixels of color interspersed throughout), you'll get lots of sharpening in totally inappropriate places. The result? Your photo will look worse instead of better. I tend to start with a threshold of 5 and lower it if I have a very clean, low-noise image. I raise it as high as 10 if my image is kind of dirty.

Also, keep in mind that these values are interrelated. If you use a high Clipping value because your picture was taken in low light and suffers from some digital noise, you may want to increase the Strength to 150 percent to beef up the sharpening effect.

Experiment with the Unsharp Mask tool and you'll find that you can improve pictures you thought were sharp, but were actually a bit soft. It's especially handy for images you plan to print on ink-jet printer.

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