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Wednesday, August 1, 2007

Sharpening Techniques

A strong use of Radius in Unsharp Mask may help your nature images

Digital Horizons: Sharpening TechniquesTop lenses often have a level of brilliance that gives an image snap and sparkle that lower-priced lenses can’t match, even though a given lower-priced lens may have similar resolution (sharpness). Brilliance isn’t something you necessarily want when shooting portraits (it makes the skin texture more obvious), but it really does help landscapes and other nature photography.

As Radius increases, tiny specks of bright highlights intensify—this is the halo "problem" on a small scale, but in this case, it helps the photo. This shows up in the overall image on your screen as long as you have a large enough monitor with high monitor resolution. This is also why I don’t enlarge the main image to 100 percent—you miss the brilliance effect. I’ll watch the 100-percent image preview in the Unsharp Mask box to be sure I’m not having halo problems, however.

An important caution: Note that the numbers used for Radius aren’t high. Most of the time, I use 1.5 or lower (because I’m dealing with images from sensors less than 10 MP in size).

Unsharp Mask is the basic sharpening tool to know and use. Smart Sharpen can be used in Photoshop, plus there are Photoshop plug-ins that help, which I’ll discuss in a moment. Still, Unsharp Mask (or USM, as it’s often called) is an excellent sharpening tool because of its control.

USM has three basic controls: Amount, Radius and Threshold. They work together—if you’ve seen many formulas for USM, you’ll notice that as one changes, the others change, especially Amount and Radius, as noted above. Any number for Amount, for example, is meaningless without knowing a Radius. Here are some ideas on how to set them:

• Amount is the intensity of the sharpening. It needs to be higher when detail is fine, Radius is low, and any time you need more
sharpness from a subject (again, you can’t get sharpness from a blurred photo and increasing Amount will make such a photo look ratty). You’ll generally use higher settings for highly detailed subjects, such as rocky landscapes, and lower settings for gentle subjects, such as selective-focus flowers. I like an Amount of 130 to 180 for nature subjects depending on the actual subject and based on the Radius I like
to use as described above.

• Radius is described in detail earlier in this column. But to summarize, I generally use a Radius between 1 and 1.5 for most images, going up to 2 for a very large print (with a file size of more than 30 MB) and going to less than 1 for small prints (with a file size of 6 MB or less).

• Threshold affects when the sharpening occurs based on contrast of an edge. A low Threshold gives high sharpness, but it also sharpens noise and other artifacts. A high Threshold reduces small detail sharpness. For most digital cameras, I use a Threshold of 3 to 4. If I find any noise problems, I increase this to 10 to 12, but never higher. I look for grain in scanned images to set Threshold with film, again never going above 10 to 12, but using low numbers when I can.


Unsharp Mask Controls
Setting What It Does Range To Try
Amount Controls degree of sharpening 130-180
Radius Where sharpening occurs at the pixel level 1-1.5
Threshold Affects the sharpness (and appearance) of noise 3-4 (most) to 12 (noisy)

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