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Using High-Pass Filtering to Improve Image Clarity

(September 2007) posted on Tue Sep 04, 2007

Unsharp masking is a useful method for enhancing digital images. Learn about different ways to apply this tool and other Photoshop features to bring clarity and vibrancy to your graphics.

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By Mark A. Coudray

Most prepress technicians are aware of unsharp masking (USM) as a method to compensate for substrate and printing degradation and as a way of improving the message of the artwork being reproduced. In this month’s column we’ll delve deeper into several variations of this technique that can dramatically improve the visual quality of images, whether they are viewed close-up or at a considerable distance.

The power of unsharp masking goes well beyond the basic Adobe Photoshop embodiment, which is accessed by going to >Filter>Sharpen>Unsharp Masking. Sharpening is done on many levels. We can apply it over very narrow ranges of tonal and color values, as well as within selections. A full exploration of all sharpening options goes well beyond this column. In fact, several of the better books on Photoshop devote as much as 100 pages to the subject.

Applying USM can be tricky. The novice user can easily get carried away and go too far. A little goes a long way, but its absence leaves you feeling that something is missing. The basic idea is to improve the tonal and color breaks in an image by lightening the light parts and darkening the dark parts. The goal is to lend additional contrast to the image. Our eyes see this as improved definition or detail. At the lowest level, USM is nothing more than contrast enhancement over a predefined area (Radius Setting). If the range of pixels we select is too narrow, we run the risk of making the image look coarse and harsh. If the range is too great, the sharpening is minimized or becomes largely ineffective. It’s up to our trained eyes to make the decisions that lead to the most beneficial results.

Image enhancement

One of the main reasons for sharpening has to do with the printing process itself. Screen printing is not a high-resolution process, and we all know there are plenty of opportunities to degrade an image in screen printing. This image degradation presents us with the need to sharpen. In effect, we’re overcompensating for the anticipated losses that will naturally occur during the printing process. We use the same logic when we change the tonal range of an image to compensate for on-press dot gain and tone compression. We’re making up for anticipated loss ahead of time.


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