All digital cameras apply some level of digital sharpening while saving the image to a JPEG format. While most compact digital cameras offer the photographer little or no control over their sharpening process, cameras that are more sophisticated allow the photographer to alter the level of in-camera sharpening. Even then, the images often need additional sharpening on the computer. The amount varies depending on the type of subject, size of the image, and whether the image is for printing or electronic display.
What Sharpening Does
At the edge of an object, brightness changes from level to another for the object’s background. Digital sharpening increases the difference in brightness in the pixels defining the change from object to background. Sharpening does not increase resolution, which is the ability to differentiate between fine lines in an image, digital sharpening only makes the lines more obvious by highlighting their edges.
UnSharp Mask (USM)
The odd named method is the basis of most sharpening processes. A deliberately blurry mask comes from the original image, and this is mathematically combined with the original in computer image processing software to identify the object edges in the image. Then the pixel contrast is altered along the object edges making it easier for the human eye to pick the border of the objects.
In simple image processors, the photographer specifies more or less sharpening while in software that is more powerful, photographers have greater control. In image processors such as Adobe’s Photoshop and the free image processor the GIMP there are usually three variable sharpening characteristics.
- Amount This is the difference in contrast between the image pixels on either side of the edge.
- RadiusThis is the width of the edge region, often specified in pixels. The wider the edge to more noticeable it is to a viewer.
- ThresholdThis helps in deciding on what is an edge for sharpening, and what areas of the image are ignored in the USM process.
How much to Apply
Photographers have varying preferences on the best settings to use, and they vary from image to image. However, if an image is over-sharpened the pixels around the edges start to stand out and it sometimes appears as though the objects have been outlined with a pencil. The default settings in the image processor are usually a good starting point, but try applying different levels of each setting, then use the undo command to help give an idea of the effect of your sharpening level.
Fine detail in an image often needs less sharpening than larger objects and adjusting the threshold value is one way of applying different levels of sharpening in a digital image. This applies to such things as fine hair, minor skin texture, or digital image noise.
When to Apply
Most professionals sell their images to magazines with little or no sharpening applied, leaving it to the magazine staff to apply sharpening to suit their printing process. So the general rule is to leave images unsharpened until they are needed. Images often need sharpening after resizing, so leave any sharpening until after resizing.
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