Over Sharpening

Sharpening lines

FIGURE 1 The typical sharpening effect on an image is the black and white outlines around detail, with the line thickness and intensity varying depending on the settings used. The twin black and white edges are not always seen together though. Often only the black lines are visible around light parts of an image and white lines around dark, as seen in these two examples.  

These lines do not actually make the image sharper, they only give the illusion of sharpness as they add contrast to the edges. But whether this sharpening effect is beneficial or not depends on where the image is to be used.  Print and web users have very different approaches to sharpening, and picture libraries will often specify that no sharpening is to be used at all. Regardless of whether you are for or against, these notes are intended to help in detecting the various effects.

Sharpening spots

FIGURE 2  Examples of an image that has been sharpened using a high threshold setting (left), and where the print sharpen settings were applied where there was little crisp focus detail to start with.

If the sharpening uses a reasonably high threshold setting then the black and white outlines do not appear at certain levels. But in other areas of the image the lines can suddenly come into effect, such as in the image on the left. This can result in a series of speckled uneven lines of varying thickness, almost as if they had been painted in by hand with a brush. If the sharpen threshold setting is made even higher then the effect starts to diminish, to the point where no sharpening is seen at all.

Problems in sharpening do not necessarily mean the use of low grade equipment. The image on the right had print sharpen settings applied where there was little crisp focus detail to start with. The effects are an exaggerated form of the sandpaper problem; more like a plague of black spots instead.

Sharpening grain

FIGURE 3 The two images shown here are from the same frame, but scanned by different people. 

The intention of sharpening is to exaggerate the difference in contrast around image detail. This can work well where an image does have detail to sharpen. But where areas of an image are out of focus, such as background, or where there is effectively no detail at all, such as in blue sky, then sharpening can be a real problem if done incorrectly. As there are no hard edges in these areas, the result can be to only sharpen what does exist here, namely film grain and scanner noise. Rather than seeing the common black and white lines, the visual effect is more akin to sandpaper.

As shown in the two scans of the same image, above, there are ways to avoid this sandpaper effect, but all too often blanket settings are used that can ruin an image. Photographers use various methods to draw the centre of focus to the image subject; keeping only the subject in sharp focus for example. The same approach can help when applying sharpening, otherwise you run the risk of drawing attention away from the intended subject.