Unsharp Mask

September 5, 2011 | 2 Comments |

Precise sharpening can improve almost any image. It helps to know when to apply it, what type of sharpening to apply, how to apply it and where to apply it. Forget the filters Sharpen, Sharpen More and Sharpen Edges. They’re just default settings of Unsharp Mask. Even Smart Sharpen offers few advantages over Unsharp Mask; it’s particularly useful for compensating for trace, but not substantial, amounts of motion blur. My advice? Start with the classic and master it.

Why is a filter that makes images appear sharper called Unsharp Mask? In silver-halide-based photography, unsharp masks are made with out-of-focus negatives that are registered with an original positive image. During exposure, the blurring adds contrast around contours, making images appear sharper. Digital unsharp mask works the same way; it uses blurring algorithms to add contrast to contours, again making images appear sharper.

What are the ideal settings for Photoshop’s Unsharp Mask filter? There are no ideal settings that will accommodate all images—or image makers. Settings will be influenced by resolution, ISO, subject and practitioner. As creative sharpening is primarily an aesthetic decision, individuals are likely to prefer different amounts and types of image sharpness. When it comes to the effects Unsharp Mask generates, there’s a general range of believability most viewers share, but whether you play it safe or push the envelope is entirely up to you. You can craft your own sharpening style. To do this, you have to know how the tool works and what to look for.

What are the controls Unsharp Mask offers? Unsharp Mask offers only three controls: Amount, Radius and Threshold. What do they do? Amount controls contrast; a higher setting will create a brighter halo, darker line and contrastier texture. Radius controls how thick halos and lines get. Threshold suppresses the effect in adjacent pixels, based on their relative luminosity; with a very low setting, only adjacent pixels that are very close in color will be affected; with a very high setting, many more color values will be affected.

It’s one thing to hear this. It’s another to see it. To test the filter for yourself, take these six easy steps.

1. Apply the filter to an image containing a range of textures; set all three sliders to their lowest settings.

2. Raise the Amount all the way to 500%. Nothing will happen because it’s not the effect; it modifies the effect.

3. Raise the Threshold all the way to 255; then return it to 0. Nothing will happen because it’s not the effect; it modifies the effect.

4. Raise Radius. You’ll see a dramatic change in your image. What will you see? Bright lines (halos) and dark lines (lines) will appear and grow thicker. Texture will increase. Contrast will increase, particularly around contours and with respect to texture, which may make luminance noise more pronounced. Saturation will increase; color noise may begin to appear.

5. Now move the Amount slider back and forth; you’ll see the halos and lines increasing and decreasing in contrast.

6. Move the Threshold slider back and forth; you’ll see the effect dropped out of a varying range of adjacent values.

While you’re sharpening, keep an eye on these image elements: contours or halos and lines (hard or soft, thick or thin); texture; noise (light/dark or color); contrast; and saturation.

Now that you know how the filter works, how to control it and what to look for, what effects should you consider? There are two primary ways to apply Unsharp Mask: use a low Radius or use a high Radius.

Low Radius applications of Unsharp Mask strengthen the contrast of contours more than their thickness and often can accentuate texture aggressively, for better or for worse. Start with an Amount of 500%. Raise the Radius until it produces an effect that’s unnaturally contoured and textured, then pull back slightly. Reduce the Amount to subdue the effect somewhat until the effect seems convincing. Use a minimum Threshold setting or else the effect may be suppressed unnecessarily and sometimes unnaturally. The idea behind this classic effect is to create a very intense line for maximum effect and to make it very thin so the eye can barely resolve it. Use a maximum Amount, a very precise Radius and a minimum Threshold. When using this effect on high-resolution files, because there are more pixels in a high-resolution image, Radius settings will be higher, you’ll be able to set them more precisely, and you’ll be able to use higher amounts. Higher-resolution files can be sharpened more precisely.

High Radius applications of Unsharp Mask strengthen the thickness of contours more than their contrast and don’t accentuate texture aggressively. Start with an Amount of 100%. Raise the Radius until it produces an effect that’s unnaturally contoured and textured, then pull back until the effect seems convincing. Use a minimum Threshold setting or else the effect may be suppressed unnecessarily and sometimes unnaturally. High Radius effects are often less aggressive or dramatic than low Radius effects. For this reason, they’re often combined together in multi-pass sharpening routines.

You can sharpen an image multiple times and achieve a different effect than sharpening an image only once. How? First apply one type of sharpening and then the other. Typically, high Radius settings are used before low Radius settings, taking care not to create sharpening artifacts in the first pass that will be accentuated adversely in the second pass.

Find more sharpening at Digital Photo Pro.

Read more with my digital photography ebooks.

Learn more in my digital printing and digital photography workshops.


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