Unsharp Mask

Use Unsharp Mask (USM) filter to sharpen your images (Well, the name of the filter is quite misleading). Digital images are composed by many pixels; the smallest unit is 1 pixel. You can’t change the color for half pixel. To change the definition of edges (many adjacent pixels) in image, can enhance the contract. In Photoshop CS2, Filter > Sharpen > Unsharp Mask…, adjust the Amount, Radius and Threshold.

Tips: For image with thousands of pixels in width, Amount: 85%, Radius: 0.8-2 pixels, Threshold: 0 levels; image with hundreds of pixels in width, Amount: 75%, Radius: 0.5-0.8 pixel, Threshold: 0 levels, or Amount: 200-300%, Radius: 0.3 pixel, Threshold: 0 levels.

Tricks: Apply Amount: 10-25%, Radius: 55 pixels, Threshold: 0 levels on a large landscape image see what happens.

References: "Sharpen and Sharpen More Focus the image and improve its clarity by increasing the contrast between adjacent pixels where there are significant color contrasts, usually at the edges of objects. The Sharpen More command sharpens with a stronger effect than the Sharpen command. Both commands are automatically applied to the image. Unsharp Mask Sharpens the mid- to high-contrast edges in the image without enhancing noise. This command locates adjacent pixels that have a difference in lightness values that you specify and increases their contrast by an amount you specify. This command is typically used in professional color corrections." Source: Paint Shop Pro

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