When it comes to photography with digital SLR cameras I always recommend setting the in camera sharpening setting to zero. Doing this gives you the highest quality jpg file to work with (if you are not shooting in RAW mode) and also gives you much more creative control once you are in Photoshop (PS). While Raw conversion programs can be used to apply some "capture" sharpening to the image, I almost always find that I need do some additional sharpening before I print an image or post it on the web.
Take note that it is usually best not to sharpen an image until it is ready to be printed, posted on the web or made ready for presentation in some other form.
The ultimate sharpening tool - unsharp mask:
When it comes time to sharpen an image you can forget about the “sharpen” and “sharpen more” tools in PS. The tool to use is unsharp mask (USM) (Filter > Sharpen > Unsharp Mask). While somewhat tricky to get the hang of, this tool enables the ultimate in sharpening control.
Before sharpening I like to convert images over into LAB mode (Image > Mode > LAB) and then select the lightness channel to sharpen from the channels window. The reason I do this is that it cuts down on color artifacts that would otherwise be emphasized in the sharpening process.
A general rule is that smaller files will require less sharpening. With a full size image out of a 10-15 megapixel camera I would suggest setting the sliders to 100-120%, 0.5-0.8 and 3 as a starting point. You can then adjust each as necessary until you have achieved a result that is pleasing to you. Do note however that over sharpened images are definitely undesirable. And always preview your images at 100 percent when sharpening. Once satisfied you can convert your image back to RGB colour.
I stated above that I never sharpen an image before I am ready to print or display it. That is not entirely an accurate statement. In some cases I will selectively sharpen an area if I feel that it improves the image. Most of the time this involves sharpening an area around the subjects eyes. To do so simply select the area that you want to sharpen (see article on making selections). Then use the refine edge tool to feather the selection. Another option would be to apply sharpenning to a second layer and then adjust which parts of the image are sharpenned using a layer mask (see seperate article for tips on masking)
Sharpening for printing:
When I sharpen an image to send to the photo lab I always sharpen it more than I would normally. The reason for this is that the printing process inherently involves some loss of sharpness. A little tip that I picked up a while ago was to view my images in PS at 50% when sharpening an image to send to the lab. The result is that the image gets over-sharpened just enough to balance out the softening process of the printing.
Sharpening for the web:
Sharpening an image for the web works the same way as described above. Just make sure that you adjust the vales of the USM filter accordingly. I usually present my images on the web as 4 x 6 inch images at 120 pixels / inch. To sharpen an image of this size try out settings in the neighborhood of 100-120%, 0.3 and 1 as a starting point.