Often, an image direct from a digital camera may be a little soft, and will benefit from a little judicious sharpening. Indeed, you are far better disabling any in-camera sharpening and sharpening images in Photoshop after the event, as this gives you far more control over the final degree of sharpness in the image.
Your one-stop-shop for sharpening images in Photoshop is the Unsharp Mask filter. This wonderfully powerful sharpening device is derived from an age-old printing process which improved the sharpness of an image printed from a film negative. Because its effect can be so powerful, using the Unsharp Mask filter demands some restraint, and you are far better using it subtly a couple of times rather than going for a single big hit.
One point to bear in mind is that sharpening any image should be the very last action within Photoshop before saving the final version of an image, and should be done after all other image manipulation has been completed.
The Unsharp Mask filter works by identifying the edges in an image and subtly exaggerating the contrast between the pixels which lie either side of this edge. In an unsharp image, the boundaries between these pixels are soft and indistinct. When using Unsharp Mask, you instruct Photoshop to narrow these boundaries, and, with the various controls, tell it how wide and how much contrast the boundaries need to have before the edge is sharpened by the filter. Essentially, the resulting sharpness is actually ...