By far, the tool that offers the most control and yields best results for sharpening images is the Unsharp Mask tool. Unsharp Mask introduces sharpness by evaluating and adjusting the contrast difference between pixels along an edge. These pixels along this edge are made lighter (and/or darker) giving the illusion of sharpness. Unsharp Mask offers three controls:
- Radius. The Radius setting affects the size of the edge to be enhanced and determines how far out from an edge contrast is increased. This is typically the most critical setting as high amounts can cause unwanted halos, especially in flesh tones. A good starting point for the Radius setting is 1-2 pixels.
- Amount. Amount determines how much darker and how much lighter the edge borders become where the light and dark pixels meet. A good starting point for the Amount setting is 60%.
- Threshold. Threshold is the difference in tonal values necessary for pixels to be affected by the sharpening. Low values sharpen more because fewer areas are excluded. Higher threshold values exclude areas of lower contrast. A good starting point for the Threshold setting is 3-4 levels.
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Finally, always view images for sharpness in Photoshop at even zoom views, such as 50%, 75%, 100% and so on. Viewing images at any other zoom rate will make the image appear sharper than it actually is when it prints.