What is USM?

USM stands for Unsharp Mask (or Masking) and it is a technique used to sharpen a digital image by increasing the contrast along the edges of details in the image. An increase in local contrast causes an image to appear sharper to our eyes. USM has it’s roots in traditional photography darkroom work when the only way to sharpen a slightly soft image negative was to make an out of focus copy of the original negative. This copy was then sandwiched in register with the original negative and the print made. It was found that this increased the apparent contrast of the image producing a better print. The copy negative became known as an Unsharp Mask and this is where the digital equivalent gets it’s rather weird name from.

The USM dialog settings

To use USM you need to understand a little about the settings in the Photoshop USM dialog so here goes...

The Amount value is the percentage of contrast increase applied to the pixels found on each side of an edge (i.e. any line or detail). For example, if you have an edge in the image where on one side of the edge there are pixel values of, say, 30% and on the other side pixels of 50% (not too much contrast in this example), using an amount of 100% (or whatever) will INCREASE THE DIFFERENCE in contrast of these pixels by this amount. So, 50 minus 30 is 20% difference. Increase will be to 40% difference and so the final pixel values will be 20% and 60%.

The Amount value you use will depend on how much data i.e. the file size in Mb, you have to play with. For the same amount value, small images may show the dreaded 'white haloes' around details whereas a large image using the same amount won't have a problem. For small images use your eyes to judge the screen image because that is usually how small images will be viewed and for larger images that you want to print aim for a 'crunchy' look (not very technical but you will know it when you see it!). Remember that printers are working at higher resolutions than screens so any apparent roughness in the sharpened image will 'smooth out' in the print and not be obvious.

The Radius value is how many pixels away from a found line will be adjusted. A radius of 1 will only use one line of pixels each side of the found edge. This will produce a much more subtle sharpening and is suited to highly detailed images where the last thing you want is to lose subtle details. Higher values produce much more aggressive effects. Again, let your eyes be the judge.

The Threshold value is the minimum pixel value that will be used in the sharpening. With a threshold of 3 any pixels with the value 3 or less will be ignored. This allows you to control the subtle details in shadows. Usually, higher threshold values reduce the impression of sharpening because the sharpening will be applied to fewer pixel values.

Although there is no one answer to which settings are best, you need to apply the above knowledge to your chosen image. Images with lots of details, especially in shadow areas, may need less Radius but more Threshold to produce an increase in sharpness without too much loss of detail. Images with lots of smooth tonal areas, e.g. blue sky can stand higher Amount values and lower Threshold with Radius being up or down. However, the downside is that you may create a 'grainy' impression in the tones and possibly a little posterisation. Again, use your own eyes to judge!

There you have it; you now know what USM is and how the settings affect the sharpening results.

Photoshop for beginners - What do the USM settings mean?

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