There is a widely-held misconception that image manipulation is good for sharpening blurred photographs. The truth is that while a computer can do most things to an image, rescuing a blurred shot remains impossible. When an image is out of focus, little of the detail – and none of the fine detail – is recorded. No amount of manipulation can bring this back. However, image manipulation applications do feature Sharpen tools that can be used to add sharpness to an image that was not critically sharp in the first place.
Remember that while these tools can improve an already good shot, used without care, they can compromise overall quality. Often, you do not enhance an image by digital sharpening, but rather make its digital origins all too obvious.
Sharpening tools work by identifying edges in your image. In an unsharp image, such boundaries tend to be comparatively wide and soft. These transitions are then narrowed, to give the perception of sharpness.
Sharpen filters
You may find many Sharpen filters in your application. A typical set might include Sharpen, Sharpen More, Sharpen Edges and Unsharp mask. Sharpen More applies a sharpening effect about four times as great as Sharpen. Both are somewhat crude in their effects, as is Sharpen Edges. In each case, there is a tendency to introduce artefacts that, at best, reduce contrast and give a flat image. Worse still, they can introduce very obvious artefacts.
Unsharp mask
Despite the name (which originated from a traditional photographic darkroom technique) the Unsharp mask provides a controllable way of image sharpening.
Using the Unsharp Mask is an inexact science. Though there can be guide figures for the parameters, a degree of trial and error is required to find the optimum values.
Because sharpening tends to exaggerate artefacts (or introduce new ones), subsequent image manipulations such as tonal changes, saturation increases or adjustments can make these more prominent. It’s a good idea, then, to make sharpening your final adjustment prior to printing or saving your image. Mode changes (say from RGB to CMYK) can make sharpening more obvious, so again, sharpen after any such changes.
Examine your image once sharpened, both overall and in detail, to assess the effect. A sensible precaution (as with any major image manipulations) is to work on a copy of your image so that you can revert to the original should you later discover a problem.
Tips
In portraits, you aim to limit the amount of fine detail (such as skin texture) but sharpen the larger and more pronounced elements (such as eyes and lips). If you were trying to sharpen a landscape (or, more particularly, a cityscape), you might want to retain as much fine detail as possible. In these cases you would set (as a starting point) your parameters as: Threshold, 0 (to sharpen everything); Radius, 2; Amount, 150 percent.
