[ohenry]

First, sharpness and sharpening are two separate terms. Sharpness is the result of focus, exposure, lens quality, film/sensor, camera movement, etc.

Sharpening is a process applied to digital images, whether scanned from a slide, print, or direct image from a digital camera. It is a process of increasing the contrast of the pixels where there is an edge or natural difference in contrast, brightness, or color. Sharpening is a "destructive" process in that it is applied directly to the image pixels and permanently alters pixel contrast values. Too much, or repeated, sharpening can and will degrade the image.

There are many ways to sharpen an image, some better than others for given circumstances and each having their own merits. Digital cameras apply a given amount of sharpening, which in most cameras can be turned up or down or even off. I prefer to shoot RAW and never sharpen from the camera. Often when you see someone say that their XYZ consumer point and shoot gives better images than their $1500 dSLR, it's because of the amount of sharpening (and other parameters) applied by the camera. Post sharpening is generally preferred if you plan on doing post production work. Because of the destructive process of sharpening, it is generally applied after tonal adjustments are made. Bruce Fraser feels that sharpening should be applied in a 3 step approach. First for the softness of the capture device, then again for creative sharpening applied to specific areas of concern, and finally sharpening that is targeted to the final output. Personally, I follow his method and use his sharpening software 99% of the time that I sharpen.

Unsharp Mask is probably the most popular method of sharpening. In Photoshop (not sure of other programs), you can control the amount of sharpening, which increases the contrast of pixels by either lightening or darkening them. If too high, you'll see halos or fringing at the edges. Radius is another control available for USM. This setting determines the width of the sharpening halos. Obviously, the wider the width, the more pronounced the effect. If too wide, the image will appear oversharpened. Radius and amount work together back and forth...when you increase one, you should decrease the other. Threshold is the final setting control. It controls which pixels the USM filter will consider. It has a value of 0-255, so if you enter a value of 8, the filter will sharpen a pixel only if there's a difference of at least 8 tonal levels between it and the pixel next to it. Typical threshold values range from 0.5 to 8. I'm not really sure why threshold goes as high as it does as higher levels rarely will help the image. I've seen many comment that good starting points are: Amount of 300 then use radius and threshold to back off the intensity until you reach something that pleases your eye.

Other methods of sharpening include edge sharpening, high pass filter sharpening, and, of course, plug ins such as Fred Miranda's, Pixel Genius, and others.

The amount of sharpening you apply depends on the size of the image and the output medium. What looks good on your monitor may not do well on an inkjet printer and what works for your inkjet printer may not work for commercial print machines.

Hope I answered your question somewhat. Obviously, this is a much more in-depth topic than I've outlined briefly here.