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May
2000

Lookin’ Sharp

There are a wide variety of scanners and digital cameras available to today’s computer users, but no matter how good your scanner is or how crisp your original is, your scans will lose some of their sharpness when they are digitized. Even scans from high-end drum scanners can benefit from additional sharpening. Images can also lose some sharpness when they are printed to laser or inkjet printers that lay out dithered dot patterns. When an image is resampled, it will also lose sharpness. The main tool to use to counteract this blurriness is Adobe Photoshop’s Unsharp Mask (or USM) filter (Filter: Sharpen: Unsharp Mask). I must point out that the Unsharp Mask filter will not make an out-of-focus image in focus, the best solution for that situation is to reshoot your photograph.
The human eye is very good at seeing edges of objects—sharp lines between two colors. The more contrast between the edges, the shaper the image appears to us. The way that the Unsharp Mask works is to analyze the tonal differences between adjacent pixels; the more contrasting they are, the more USM increases the contrast. This results in a "halo" around the edges which increases the images perceived sharpness. The USM can also have the undesirable effect of exaggerating textures, noise and JPEG artifacts. The filter offers controls to help strike a pleasant balance between sharp and hideously oversharp. When sharpening you want to view the image at 100% to get the most accurate view of the effects.

Fig. 1

There are three controls in the Unsharp Mask dialog box: Amount, Radius and Threshold. Amount and Radius are inversely related. If you have a high Amount, you should lower the Radius, and vice versa. I like to start at a fairly high Amount like 200, and a lower Radius of about .7. For images that are going to be on the internet, I will use a setting around 300 and .3. The settings will change according to the resolution and characteristics of your specific image. Typically, I usually set the Threshold at 2. Threshold helps to prevent unwanted sharpening of noise, texture and artifacts. It basically tells USM to ignore pixels that are less that 2 tonal levels apart.
Fig. 2
The Unsharp Mask is a powerful and useful tool. I use it every day for all of the images that I work on. Experiment with the settings to find what works for you. Use it and you will be pleased with the sharp looking results.
Paul Vaughn is the Director of Digital Services at River City Silver, the premiere photographic and digital imaging laboratory in San Antonio and South Texas. Mr. Vaughn is a sharp lookin’ graphic artist. His web site is at http://pv.home.texas.net, he can be contacted at paulv@mac.com.


This site and all images and text contained in it are ©2005 Paul Vaughn.
(Unless otherwise noted)
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This site and all images and text contained in it are ©2006 Paul Vaughn.
(Unless otherwise noted)
Questions? Problems?