July 22, 2011

Using Unsharp Mask

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Post & Photo by Joe Farace – Follow Joe on Twitter

The first thing that your eyes notice about a photograph is image sharpness, followed by brightness and then warmth. There are degrees of each but especially sharpness. Depending on how they are captured, some digital files may be sharper than others.

One of the biggest advantages digital imaging has over traditional film photography is the ability to sharpen images. Many image editing programs, contain a Sharpen command that typically works by raising the contrast of adjacent or edge pixels, but sometimes this technique increases sharpness at the expense of overall contrast. Some photographs can handle additional contrast before loosing highlight detail, while others can’t.

Instead of using the Sharpen command, a better way to increase an image file’s sharpness is by using the unlikely named and wonderfully practical Unsharp Mask (Filter > Sharpen > Unsharp Mask) command found in Adobe Photoshop and other image-enhancement software. Unsharp Masking is the name of a traditional darkroom and prepress technique where a blurred film negative is combined with the original to highlight the photograph’s edges. In digital form, it’s a more controllable method for sharpening an image than using standard Sharpen commands.

The Unsharp Mask dialog box offers three sliders let you control the amount of sharpness: The Amount slider controls the percentage of sharpening that will be applied to your photograph. Don’t be afraid to apply more than 100 percent to higher resolution files but lower resolution images will fall apart if larger amounts are used. The Radius value will vary depending on subject matter, final reproduction size, and output method. For high-res images, a Radius value between 1 and 2 is usually recommended. A lower value sharpens only the edge pixels, whereas a higher value sharpens a wider band of pixels. The Threshold slider lets you determine how different the sharpened pixels are from the surrounding area before being considered edge pixels and sharpened by the filter. On a scale of 0 to 255, using a threshold of 4 affects all pixels with tonal values that differ by 4 or more. If adjacent pixels have tonal values of 128 and 129, they’re not affected.

Tip: Here’s a useful technique for dealing with color fringing that might occur when applying Unsharp Mask to certain kinds of image files. After applying the sharpening to the image file, go to Photoshop’s Fade command (Edit > Fade Unsharp Mask) that appears only after a filter is applied. Don’t change the Opacity setting—leave it at 100%—but select Luminosity from the pop-up menu. Any glaring color artifacts should then be gone!

Joe Farace is the author of “Studio Lighting Anywhere” the second book in a trilogy from Amherst Media. It’s available on Amazon.com. The third book, “Available Light Glamour” should be available “real soon now.”

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