Image sharpening tips

The essence of the sharpening operation is to increase the contrast of the edges of the image details, which helps our eyes to see the details of the image, so that the image appears sharp and clear, the picture is clear, which is a necessary condition for all good quality print photography. Images that have been copied directly with the scanner, if they have not been trimmed, may look a bit monotonous and obscure, so we often need to sharpen the image after it has been processed.

The most professional sharpening method we recommend is Photoshop's unsharp masking (USM), which provides the most complete control of image detail emphasis. It provides three control parameters:

Radius: It is used to determine the width of pixels for edge enhancement. If the radius is 1, the entire width from light to dark is two pixels. If the radius is 2, two edges are on each side. Pixels, then the entire width from light to dark is 4 pixels. The larger the radius, the clearer the difference in detail, but it also produces halos. A reasonable threshold should be set to the image scanning resolution divided by 200. For example, 200 sp uses 1  0 in the scan and 1  5 for the 300 spi image. This produces a 1/50 to 1/100 inch halo around each edge, which is large enough to provide the ideal Sharpening effect.

Amout: This parameter can be understood as the intensity or amplitude of sharpening. For general prepress processing, setting 200% is a good start and then adjusted as needed. Too large number of images will become false.

Threshold: It determines how much contrast the neighboring pixel borders can be sharpened, and below which the contrast does not sharpen. Threshold setting is a key parameter to avoid spots and pitting caused by sharpening. After correct setting, the image can be maintained in a smooth natural tone (such as a pure blue sky in the background). The contrast of the details of the changes can be emphasized. In general prepress processing, we recommend values ​​of 3 to 4, more than 10 are undesirable, they will reduce the sharpening effect and make the image look ugly.

In short, the sharpening parameter adjustment must be able to better reproduce the details of the image without creating new problems: such as spots and pits. If you are an experienced processor, you can also perform appropriate local sharpening based on the image content to achieve special artistic effects.