9.6 SHARPENING IMAGES

Learn To Draw

Learn To Paint

Photoshop Elements

Animation Menu

Art Principles

Art Appreciation

Most photos captured with a digital camera or a scanner contain some blur that can be corrected using a process called unsharp masking (USM). In fact, the need for this process is so common, many high-end cameras and scanners have embedded software that does it automatically. Even if it's been done once to an image, it may have to be done again if you resample the image. You'll find that this procedure, done with an Unsharp Mask filter can make a tremendous difference in photos that you print or post on the Web.

So how does the Unsharp Mask filter work? First it locates edges in the image by looking for pairs of adjacent pixels that have a specified difference in brightness (threshold) and increases the contrast of those pixels by a specified amount. The change affects not only the pair of pixels, but also any that fall within a specified radius.

• The threshold is the difference in brightness between two pixels before they are considered edge pixels and are sharpened by the filter. If you set the threshold value to 0,  all pixels in the image will be sharpened. In some cases, such as images with flesh tones, this can introduce noise. In these cases, you might want to experiment with higher thresholds, perhaps between 2 and 20.

• The amount is the percentage which contrast of the edge pixels is increased. A good starting point is around 100%.

• The radius is the number of pixels around the edge that are sharpened. To begin, you might try a radius between 1 and 2 pixels.

When sharpening an image, keep in mind that the effects of the Unsharp Mask filter are more obvious on the screen than in high-resolution output. To get a feeling for the effects, do some experiments and sample printouts to see what settings work best for you.

An unsharpened image of a subflower blossom.

Sharpened image.