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Digital Artifacts…

Sharpening and Unsharp Masking

artMike
The file as is, needing a little sharpening.

Some loss of perceived sharpness occurs at capture stage with any sensing device. There is some chemical compensation for this in film capture due to increases in edge contrast. Digital techniques which produce similar effects, have been used with scanners since the inception of digital imaging. Various digital sharpening routines are available but most depend on the addition of edge contrast in one form or another. In the days when all graphic arts procedures were film based, a technique was developed for increasing edge contrast by first making an unsharp monochrome negative and then generating a soft but contrasty positive which was then sandwiched with the original.


artMikeUn
The same file with an excessive amount of unsharp masking applied. Note the halo around the hair and harsh facial highlights. Digital cameras with small sensors are likely to produce similar artifacts, though not as pronounced as this example.

This type of analog unsharp masking can be emulated very well in digital sharpening filters which today carry the anomalous description of "Unsharp Masking.".Because of the nature of current CCD image capture, consumer level digital cameras use Unsharp Masking routines after the image data has been interpolated. There is no choice by you, the user, as to how this is done or how much is applied. Unfortunately there is also no fix for overuse of Unsharp Masking, other than buying another brand of camera. You can see from our example, done in software, what the effects look like. As more Unsharp Masking is applied, and as it is applied to a wider and wider edge radius, a characteristic shadow / halo effect becomes apparent. It will often appear as ghosting or black line around hard edges. You will not see radical effects like this in original camera images, but now that you know what it looks like you may well recognize its presence in small amounts, particularly in images from cameras with small sensor arrays.

 

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