The Unsharp Mask filter in Photoshop CS6 provides a sophisticated attempt to duplicate a sophisticated photographic effect called (you guessed it) unsharp masking, in which two sheets of film are sandwiched together to create a final image.

One sheet is the original film negative (or a duplicate), and the second is a positive image (the “normal” photograph) that’s blurred slightly. When the two are put together, the light and dark areas cancel each other out, except at the edges — because of the blurring of the positive mask, which causes the edges to spread at those points.

Unsharp masking is a tricky procedure in the darkroom. It’s much more precise in the digital realm because Photoshop can easily control the width of the areas to be masked, as well as a relative brightness level to use before beginning to apply the masking effect.

In the Unsharp Mask dialog box, you can find two of the same controls that you have with Smart Sharpen — Amount and Radius. You also have another option, called Threshold. Threshold controls the difference in brightness that must be present between adjacent pixels before the edge is sharpened. That is, you need to have a distinct contrast between adjacent pixels along an edge in order to sharpen the edge.

Your choices range from brightness values of 0 to 255. Selecting a low value emphasizes edges with very little contrast difference (which is usually what you want). You’re generally better off leaving this control at 0 unless your image has a lot of noise. Higher values force Photoshop to provide edge sharpening only when adjacent pixels are dramatically different in brightness.

Increasing the threshold too much can cause some harsh transitions between sharpened and unsharpened pixels.

In most cases, the Amount and Radius sliders are the only controls you need to use. Threshold is most useful when the first two controls create excessive noise in the image. You can sometimes reduce this noise by increasing the Threshold level a little.

Sharpening always increases contrast, so keep this in mind if you plan on adjusting the contrast of your image with other commands and tools. Sharpening is usually one of the last commands you apply when correcting and enhancing your images.