Output sharpening

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Output sharpening should be the very final process you apply to an image, before converting it to the final colour mode, which would typically be CMYK for an image destined for commercial offset litho print.

You should save an archive image before beginning the final resizing and sharpening process, as this process results in an image optimised for a particular printer and size, and from which it is not possible to recover an image for resizing.

Prior to output sharpening, you need to resize the image to the correct size for your printer. If you are printing to a contone printer, then the image should be the correct size at the same PPI resolution as the DPI resolution of the printer. If you are printing to a half-tone printer then you need to oversample by 1.25-2x, in terms of image PPI resolution to output LPI resolution (note! not the output DPI resolution, which is irrelevant).

Once you have the image at the correct size, then go to unsharp masking, and apply the following formula:

Radius of unsharp masking in pixels = Viewing Distance inches x Resolution ppi x .0004

If you are in a hurry, you can do resolution in pixels divided by 200 for normal viewing distances.

Apply the unsharp masking at 100%.

This will produce an image which appears oversharpened on the screen, especially for half-tone, but will give optimally sharp images.

If you are working with a layout artist or graphic designer, ensure that they do not resize the picture in QuarkXpress or Indesign, as this will degrade it substantially. If the image needs to be resized, go back to your archive image, resize it from there, and resharpen.