What is USM?
USM stands for unsharp mask (or masking), and it is a technique used to sharpen a digital image by increasing contrast along the edges of the detail in the image. An increase in local contrast provides an image sharper to our eyes appear. USM has its roots in traditional darkroom work, as the only way, a slightly soft negative image sharpening was to make an out-of-focus copy of the original negative. This copy was made with the original negative and the pressure in the sandwich. It was found that this produce the apparent contrast of the image a better pressure. The negative copy as an "Unsharp mask" became known, and this is, where is the digital equivalent of it's rather strange name.
The USM dialog box settings
USM must use you a bit about the settings in the Photoshop USM to understand dialog box so here goes...
The amount value is the percentage of the increase in contrast applied to the pixels on each side of the edge (that is, each line or detail) found. For example, you have to where there are pixel values of one side of the border of, say, 30% and on the other page pixel of 50% (not too much contrast in this example) with an edge of the image, a lot of 100% (or whatever) these pixels by this amount increased the difference on the other hand. 50 Minus 30 20% difference is. Boost difference and so that will be 40% final pixel values 20% and 60%.
The amount value, you use depends on how much data that have the file size in MB, that you to play with. For the same amount value small images of the dreaded can 'halons white' to view details, have a problem during a large image with the same amount. Small images use your eyes to determine the image because that is where rule as small images appear and print in larger images, you want a destination for a "crunchy" look (not very technical, but you will know it), if you see it!). Note are you that printers with higher resolutions than screens work, so that obvious roughness in the sharpened picture smoothing is ' in print, and not on the you are hand.
The RADIUS value is how many pixels are adjusted by a found line. A radius of 1 is used only one row of pixels of each page of found edge. This creates a subtle sharpen and suitable for very detailed images, where the last, what is you lose subtle details. Higher values produce much more aggressive effects. Again, you can be your eyes of the judges.
The threshold is the minimum pixel value that is used in the sharpness. With a threshold of 3, all pixels with a value of 3 or less will be ignored. This allows to control the subtle details in shadows. Generally, higher threshold values reduce the impression, sharpen, because the focus is applied to fewer pixel values.
Although it is not the answer which settings are best, you need to know the above apply to your selected image. Pictures with many details, in particular in dark areas, to produce an increase in sharpness without too much loss of detail less RADIUS but more threshold. Images with much smoother tonal areas, such as blue sky may be higher amount values and low threshold with RADIUS is up or down. However, the disadvantage is that you can create a "grainy" impression in the sounds and possibly a little Posterize. Use again, your own eyes to judge!
There you have it; You now know what is USM and how the settings affect results focus. Until the next time...
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Copyright (c) Les Meehan 2010.
Les Meehan is the author of seven published Photoshop books and is a qualified teacher who has taught workshops for over 20 years.
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