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Home > Digital Imaging > Image Errors > Over Sharpening

Over SharpeningGraeme Cookson

Can an image be too sharp? Yes, certainly, if that 'sharpening' has been incorrectly implemented after the photograph was taken.

Sharpening linesSharpening spotsSharpening grain

Sharpening lines

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FIGURE 1 The typical sharpening effect on an image is the black and white outlines around detail, withthe line thickness and intensity varying depending on the settings used. The twin black and white edgesare not always seen together though. Often only the black lines are visible around light parts of an imageand 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 addcontrast to the edges. But whether this sharpening effect is beneficial or not depends on where the imageis to be used. Print and web users have very different approaches to sharpening, and picture libraries willoften 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


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FIGURE 2 Examples of an image that has been sharpened using a high threshold setting (left), andwhere 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 theimage 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 thenthe 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 righthad print sharpen settings applied where there was little crisp focus detail to start with. The effects are anexaggerated form of the sandpaper problem; more like a plague of black spots instead.

Sharpening grain


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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 areal problem if done incorrectly. As there are no hard edges in these areas, the result can be to onlysharpen 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, butall too often blanket settings are used that can ruin an image. Photographers use various methods to drawthe centre of focus to the image subject; keeping only the subject in sharp focus for example. The sameapproach can help when applying sharpening, otherwise you run the risk of drawing attention away fromthe intended subject.

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