9. Sharpness, Detail, and Noise Reduction: Getting an Edge on Your Image
The human visual system depends to a great degree on edges. Simply put, our eyes pass information to our brain, where every detail is quickly broken down into "edge" or "not edge." (Thousands of years of evolution have developed our brains to ignore most of what's going on in our field of vision and instead focus immediately on moving edges that might turn out to be a hungry tiger.) An image may have great contrast and color balance, but without good edge definition, we simply see it as less lifelike.
No matter how good your camera or scanner and how crisp your original may be, you always lose some sharpness when an image is digitized. Images from scanners and digital cameras always need a considerable amount of sharpening, though high-end scanners may sharpen as part of the scanning process. Even a high-resolution digital camera back mounted on a finely focused view camera produces images that will benefit from sharpening. You cannot solve the problem of blurry scans by scanning at a higher resolution. It just doesn't work that way.
Your images also lose sharpness in the output process. Halftoned images (almost anything on a printing press) and dithered ones (such as those printed on inkjet printers) are by far the worst offenders. But even continuous-tone devices such as film recorders and dye-sublimation printers lose a little sharpness.