Unblur your photos
April 23, 2008
"What is Unsharp Mask, and how do you use it to sharpen pictures?" is a question that we get often from readers. This tip provides the answer.
Almost any image can profit from sharpening, but this is a subtle improvement that won’t fix poor focus. The most common sharpening tool is Unsharp Mask. (The strange-sounding name comes from an old darkroom technique.)
Here is part of an image zoomed to 100 percent which reveals a soft capture.
In Adobe Photoshop, Go to Filter > Sharpen > Unsharp Mask. There should be an equivalent in most image editors.
Because the effects are subtle, you should first zoom in to 100 percent to sharpen. Some people recommend 50 percent to sharpen for printing, as there will be some softening in printing and viewing at 50 percent will cause you to slightly over-sharpen to compensate. You can view the image at different percentages in the Preview box and your screen. The preview percentage can be set just below the preview window.
Move the Amount slider all the way to the right and the Threshold slider to zero. Move the Radius slider until you just start to see halos around edges, as shown in the circled areas.
Then back off the Amount until the halos are no longer obvious. For digital files of 6-12 MP you will generally want a radius of around 1 pixel.
You can increase the Threshold to avoid sharpening noise in areas such as a sky, but then you will have to increase one or both of the other two parameters a little. Don’t use Sharpen or Sharpen More. You have no control over the parameters.Sharpening should be the last step before printing, after the image is resized, because its parameters depend on the image size. If you re-size after sharpening, the halo artifacts are still lurking just below a visible amount and could be made more obvious by image enlargement.
Diane Miller is a widely exhibited freelance photographer who lives north of San Francisco, in the Wine Country, and specializes in fine-art nature photography. Her work, which can be found on her web site, www.DianeDMiller.com, has been published and exhibited throughout the Pacific Northwest. Many of her images are represented for stock by www.MonsoonImages.com.