Sharpness is best described in the same way that Supreme Court Justice Potter Stewart described obscenity: "... I know it when I see it."

It's a blend of acutance, resolution and noise or grain. It's affected by camera shake, lens quality, depth of field, focusing accuracy, the camera sensor, the printer and a host of other things.

Once you have taken the picture, you have lost control of most of the elements that affect sharpness. But, in the world of digital images, unlike processed film, you still have control of one very important element – acutance.

Acutance, or edge sharpness, is, in fact, edge contrast. Where a light tone and dark tone come together, there is an edge where the dark tone is a bit darker and the light tone is a bit lighter. This greatly contributes to the appearance of "sharpness."

In film this happens because the developer is weakened at different rates by the adjacent dark and light tones.

In a digital image this jump in edge contrast is pixel tweaking and can be controlled by the camera or an image-processing program like Photoshop. It's invariably labeled "sharpening" because "acutancing" is a really weird word. Whether you call it sharpening or acutancing, there are a number of reasons it should be applied to most digital images.

There's just one catch. Different presentations of the same picture can require different degrees and even different types of sharpening. An image on a Web site, the huge range of halftones possible in newspaper and magazine printing, specific pixels per inch on a dye sublimation printer or different ppis and inkjet print sizes can all require different degrees of sharpening.

For that reason, my choice is not to do any in-camera sharpening (or any sharpening when I am scanning film work) and to sharpen images for specific purposes.

And, as flexible and extensive as the sharpening controls are in Photoshop, I choose to use a Photoshop plug-in, PhotoKit SHARPENER from PixelGenius, which guides me through an extremely effective but more complicated sharpening process.

Initially it provides a modest sharpening to the image based on the source of the image. Digital camera images are categorized by size and whether they need smoothing because they were shot at high ISOs. There are also special settings for extreme cases (mid-res jpgs shot at high speed and showing artifacts or the "grainless" images of scanning backs).

Scanned film images are judged by film size and whether it is negative or positive.

For both camera and scanned images you choose between four edge sharpenings, superfine, narrow, medium and wide. Many images show little difference between settings. Medium handles most. But for portraits you might want to use wide to sharpen the image without emphasizing skin texture. For pictures with important but small details you might want to try narrow. Since all the sharpening effects produce Photoshop layers and do not alter the original image, you can try different setting and toggle between them to see the difference.

Once you have set up the basic sharpening, PhotoKit SHARPENER further modifies this basic image for a specific output device. Say, for example, you are going to make an inkjet print with an image size of 12x18 inches at 240 dpi. Plug those figures into the program and it will provide additional sharpening appropriate for exactly that purpose.

Now here's the good news. Not only the initial sharpening, but the output-specific sharpening are preserved as Photoshop layers. Your original image is untouched and the strength and character of the sharpening levels can be altered and fine-tuned if you think that benefits the image.

You could add other layers for other output types or sizes. It's just that you would quickly produce a giant file that slowed your computer to a crawl. From any given image, I create an initial sharpened file and then flatten the layers. That relatively small master file is then used to produce other files for specific output purposes. They are flattened and filed along with the master file. None of the files reach the size that slows down the computer.

There is a third set of tools in PhotoKit SHARPENER. The "creative sharpeners" are used after the initial sharpening and before output-specific sharpening. They are a set of sharpening and softening brushes that can be used locally to add legibility to an image. There are special-purpose brushes that can add as much sharpness as possible to soft areas. There are "JPEG brushes" that lessen the magenta and green artifacts in the shadow areas of some cameras. And there are "Super Sharpeners" that affect the entire image and go a long way in rescuing damagingly soft images. I've used these to rescue scans from an early and not very good film scanner when they were the only records left of those pictures.

A confession: my favorite of the tools in this set are the Super Grain effects. Among the things that they can do is add the appearance of film grain to digital images. I am so annoyed by the prejudice of some museums and galleries that favors film images printed on silver paper over digital images printed by inkjet, that I love to add grain to a digital image, print it on one of the heavyweight, semi-gloss papers like Hannemuhle Fine Art Pearl or Crane Museo Silver Rag and stick it in a stack of silver images. So far, I haven't been caught. But, then again, not a whole lot of museums have looked at my images.

You can get further information on PhotoKit Sharpener from http://www.pixelgenius.com. The group that makes up PixelGenius included Bruce Fraser, the major contributor to this software. Bruce Fraser died on Dec. 16. He was the author or co-author of four important books and numerous articles on digital imaging. I never met him. But once I got past the basics, it was his books, sitting on my bedside table, that guided me through the digital labyrinth. If you wanted to know what to do with those digital images after they came out of your camera and took up residence in your computer, Bruce Fraser was the man. He made many photographers better photographers. His friends and family suffer a great loss, but the world of photography is also diminished by his absence.