Prepress Tip

Sharpening Photos

The human eye has a natural tendency to view a scanned image as "soft" or out-of-focus. You'd think that a higher resolution scan would help, but that's not the case. All scanned images need some sharpening; even those scanned on high-end drum scanners. Master printers and color separators will attest to this fact - they've been dealing with this problem for years.

"Unsharp masking" is the trade term for a standard technique that printers and color separators use to sharpen images by accentuating the differences between adjoining areas of significantly different hue or tone. The traditional technique uses a mask that's a slightly out-of-focus duplicate of the original image. When the original is rescanned with this mask, there is a nincrease in the degree of contrast at the boundaries of tone shifts; however, subtl gradation in tone and hue remain untouched. The result is increased sharpness where you would normally want it - in the most highly detailed areas of the image.

You can apply this same sharpening technique to your images with the Unsharp Mask filter in PhotoShop. Some other photoediting programs offer Unsharp Masking as well.

Here's how.
There are three settings you can choose when you select PhotoShop's Unsharp Mask Filter:

Radius refers to the dimension, in width, of every sample that will be affected by the Unsharp Mask algorithm. The formula is rather easy: the radius equals the halftone multiplier! Thus, for multimedia images (where they will always be used 1 to 1) the radius is ALWAYS 1. For print output, if your halftone radius should be set to 1.4. It's that easy. Don't listen to anyone else's theory!

Amount refers to the intensity of the Unsharp Mask effect. A setting between 100% and 200% will do. My "standard" Amount setting is 120% however; some images need more than this, and others less.

Threshold specifies how many numbers of samples in an image will be sharpened. A setting of 0 will affect every sample, whereas a setting of 50 will affect almost none of the samples. Highly detailed images such as line art, require a setting of 3, whereas portraits look best with a setting between 5 to 9. (We want to keep minor wrinkles down to a minimum, don't we?)It can take a while to come up with the right combination of settings for an image. That's why many manufacturers of scanning software now offer sharpening as an option during image capture.