How Sharpening Works

To sharpen an image, choose Filter > Sharpen. Photo-shop will present you with a submenu of choices. The top three might sound friendly (Sharpen, Sharpen More, Sharpen Edges), but ignore those because they are simply presets that enter different numbers into the bottom choice (Unsharp Mask). The bottom two filters in the list are the only ones that allow you to control exactly how much the image will be sharpened. We'll start by exploring the Unsharp Mask filter and then refine our knowledge by exploring Photoshop CS2's new Smart Sharpen filter.

The reason Unsharp Mask has its confusing name is because way back before people used desktop computers, they sharpened images in a photographic darkroom. They would have to go through a process that involved a blurry (unsharp) version of the image. This would take well over an hour (don't worry—in Photoshop it takes only seconds) and would not be much fun. The process they'd go through in the darkroom was known as making an unsharp mask, so Adobe just borrowed that term.

The Unsharp Mask filter increases the contrast where two colors (or shades of gray) touch in the image, making their edges more prominent and therefore easier to see. To easily view the effect of the Unsharp Mask filter (Figure 12.30), I'll demonstrate using two documents: one that contains only three shades of gray (20%, 30%, and 50%), (Figure 12.31)and one that is a normal photographic image (Figure 12.32). With the simple document, I'll show a normal-sized image as well as a portion of each image that has been magnified by 800%. When you choose Unsharp Mask, you'll be presented with three sliders: Amount, Radius, and Threshold.

• Amount: Determines how much contrast will be added to the edges of objects and, therefore, how obvious the sharpening will be (Figure 12.33).

  Figure 12.33. The effect of the Amount setting.

  

• Radius: Determines how much space will be used for the contrast boost that the Amount setting creates. No matter which settings you use, sharpening will produce a bright halo on one side of the edge of an object and a dark halo on the opposite side of that same edge. If you use too much, you'll be adding a very noticeable glow around the edges of objects instead of a barely noticeable halo (Figure 12.34).

  Figure 12.34. The effect of the Radius setting.

  

• Threshold: Determines how different two touching colors have to be for sharpening to kick in. With Threshold set at 0, everything will get sharpened. As you increase this setting, only the areas that are drastically different will be sharpened (Figure 12.35). If the setting is too low, then unwanted artifacts like noise and film grain will be exaggerated and relatively smooth areas might start to show texture. If the setting is too high, then the sharpening will apply to very few areas in the image, which will look too obvious because those areas will not fit in with their surroundings (which didn't get sharpened).

  Figure 12.35. The effect of the Threshold setting.

  

Now that we've explored all the options that are available with the Unsharp Mask filter, let's get down to business and find out how to apply them to an image. But before you start applying the Unsharp Mask filter, double-click on the Zoom tool to view your image at 100% view; otherwise, you won't be able to see the full effect of the sharpening you apply to the image.