"Unsharp Masking" sounds like a bit of an oxymoron. The term goes back to pre-digital prepress days when they'd make an image appear sharper by sandwiching it with a slightly blurred (unsharp) copy of the same image.
The effect then was the same as the digital effect now: the intersecting areas of light and dark in the image have an increased local contrast, which gives the appearance of increasing sharpness.
You can see the process more clearly if you grossly overapply USM to an image. The intersection contrast will be exagerated to a degree where they'll actually bloom over into each other, causing the "halo" effect so typical of oversharpened images.
Calculated the actual degree of USM to apply is actually rather complicated, since you need to take into consideration the qualities of the output device, the size of the image, and even the typical viewing distance.
It's important to remember that you're only giving an impression of increased sharpness, not actually increasing the sharpness, and that you're technically degrading the image. Also, since there are so many variables, you'll probably want to make separate printing copies of your image clearly labeled as to destination and apply USM specifically for that destination. This allows your original to remain unsharpened so you can make another copy should you need to prepare it for a different destination.
The effect then was the same as the digital effect now: the intersecting areas of light and dark in the image have an increased local contrast, which gives the appearance of increasing sharpness.
You can see the process more clearly if you grossly overapply USM to an image. The intersection contrast will be exagerated to a degree where they'll actually bloom over into each other, causing the "halo" effect so typical of oversharpened images.
Calculated the actual degree of USM to apply is actually rather complicated, since you need to take into consideration the qualities of the output device, the size of the image, and even the typical viewing distance.
It's important to remember that you're only giving an impression of increased sharpness, not actually increasing the sharpness, and that you're technically degrading the image. Also, since there are so many variables, you'll probably want to make separate printing copies of your image clearly labeled as to destination and apply USM specifically for that destination. This allows your original to remain unsharpened so you can make another copy should you need to prepare it for a different destination.
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