Friday, January 9, 2009

A word about Unsharp Mask

When it comes to digital photo editing, your best friend in the world will be the Unsharp Mask feature. Why? Because the Unsharp Mask will make your pictures sharper.

It sounds oxymoronic, but the term originated back in the early days of darkrooms and film. Someone figured out that if a negative was not quite in focus, it could be fixed to an extent by producing a slightly blurred negative of the original negative then sandwiching it with the original. The unsharp image masked some of the blur in the original negative when a print was made from the pair. This setup added contrast to the edges of the tones of the photo, giving the appearance of increased sharpness. Don’t ask me how it works. It just does. Anyway, the blurred duplicate became known as an unsharp mask. That’s all I’ll say about that.

The Unsharp Mask feature of Photoshop, et al, does basically the same thing as its filmy counterpart. It increases the contrast at the edges of a photo’s tones, giving the appearance of more sharpness. (Note: If your photo editing program only has a Sharpen feature, it's really an Unsharp Mask - it's just pre-set and cannot be adjusted.)

Your Unsharp Mask dialog box comes equipped with three variables, Amount, Radius and Threshold.

Amount is listed as a percentage, and controls how much contrast is added at the edges of tones.

Radius affects the size of the edges to be enhanced. Fine detail needs a smaller Radius. Radius and Amount interact; reducing one allows more of the other.

Threshold controls how far apart adjacent tonal values have to be before the filter does anything. This lack of action is important to prevent smooth areas from becoming speckled. Low values should sharpen more because fewer areas are excluded. Higher threshold values exclude areas of lower contrast. Sliding the Threshold setting all the way to 255 cancels all sharpening.

I nearly always run my photos through an Unsharp Mask. My default setting on Unsharp Mask is Amount 100, Radius 1.0 and Threshold 0. Depending on the photo, I’ve cranked the amount as high as 500. Since most of my photos have some fine detail, I rarely move the Radius setting. I increase the Threshold if I’m sharpening a portrait or a picture with large areas of even tones.

Open a photo and play with the Unsharp Mask settings to find a combination that works for you.

Photograph © 2008 James Jordan.

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Blogger Sharon said...

thanks! I appreciate your explanation about the unsharpen mask... I never knew what combo of numbers to use. But your advice was perfect - Now my Christmas card is snazzy!

December 1, 2009 at 8:01 PM  

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