The Unsharp Mask filter sharpens edges of the elements without increasing noise or blemish. It is the best of the sharpen filters.

Radius
Allows you to set how many pixels on either side of an edge will be affected by sharpening. High resolution images allow higher radius. It is better to always sharpen an image at its final resolution.
Amount
Allows you to set strength of sharpening.
Threshold
Allows you to set the minimum difference in pixel values that indicates an edge where sharpen must be applied. So you can protect areas of smooth tonal transition from sharpening, and avoid creation of blemishes.

Related Topics
Filter menu
Blur
BlurBlur
Blur MoreBlur
Gaussian BlurBlur
SmoothingBlur
Local AveragingSharpness
SharpenSharpness
Sharpen MoreSharpness
Unsharp MaskSharpness
Enhance FocusEdge Detection
LaplacianEdge Detection
Laplacian of GaussianEdge Detection
PrewittEdge Detection
SobelEdge Detection
KirschEdge Enhancement
Using LaplacianEdge Enhancement
Using PrewittEdge Enhancement
Using SobelEdge Enhancement
Using KirschEdge Enhancement
Edge EnhancementMin Mask
Median Mask
Max Mask
Dust and Scratches
Simple Mask