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COPYRIGHT 2005 VNU Business Media Europe
Of all image-editing filters, the unsharp mask (USM) is the most frequently used. The name originates from a darkroom technique that involves making a mask from a blurred negative, however an image doesn't have to be soft or fuzzy to benefit from unsharp masking. Most digital photos can benefit from some degree of sharpening and, whether destined for the web, an inkjet photo printer or commercial press, unsharp masking is always the final step in my image-editing workflow.
Most image editors include a USM and other sharpening filters and they all work by increasing the contrast between neighbouring pixels. The USM generally offers the most control over which pixels are sharpened and by how much.
Photoshop has a wide range of sharpening tools including the recently added Smart Sharpen which can deal with Lens and Motion blur. Paint Shop Pro's (PSP) USM filter works in a very similar way to Photoshop's with one or two differences that I'll discuss later.
USM controls
Photoshop's USM filter has three controls - amount, radius and threshold. In PSP they're strength, radius and clipping, but they do the same thing. Amount (or strength) controls the degree of sharpening applied - the amount by which contrast is increased between pixels.
In both Photoshop and PSP the amount slider ranges from zero to 500. Screen 1 shows an image with varying amounts of USM applied in Photoshop with the radius and threshold set to their defaults of 1 and 0.
How much USM you should apply depends...
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