PHOTOSHOP: Unsharp Masking (USM) Scanned images tend to look a little 'soft' when compared to their originals. The scanning process of sampling the original continuous tone artwork, photograph, or transparency, is responsible for introducing this 'softness'. One way to compensate for this is to apply a 'sharpening filter', either during the scanning process or, in the editing program, after any retouching has been performed.
Unsharp Masking The most controllable method for sharpening digitally captured images is called unsharp masking (USM). Originally a darkroom photographic technique, unsharp masking provides controls that let you adjust sharpening for particular situations. NOTE: Although it is actually impossible to make an image sharper than it already is, 'apparent' sharpness can be increased by applying a sharpening filter such as USM. This is an optical illusion. Unsharp masking works by combining a slightly blurry (unsharp) version of an image with the original. This combination results in sharp fringes or keylines in high-contrast areas (the edges, where adjacent light and dark samples are markedly different), without accenting tonal shifts in low contrast areas (areas of smooth gradation, where rapid tonal shifts would destroy the subtle transitions). This increase in edge contrast is noticed as an apparent increase in image sharpness. Since it increases the contrast in many areas of an image, unsharp masking also tends to increase the overall appearance of contrast.
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Photoshop's Unsharp Mask filter has three variables: • • •
Amount, Radius, and Threshold
Each setting increases or decreases sharpening in different areas and situations.
RADIUS The Radius setting controls how wide the fringes are on each side of an edge, in sample points. Large values result in wide fringes (big halos, lots of sample points involved); small values make smaller fringes. The size of the sharpening-induced halo affects how sharp an image looks (larger radius means more sharpening). And in excess, it's the prime culprit in most sharpening related problems. A Radius value of 1.0 results in fringes two pixels wide (four samples total, for the whole light-and¬dark cycle). Multiply the Radius setting by four to figure out how wide the halos are.
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So what Radius value should you set? For final images which will be viewed at reading distance - divide the resolution of the image by 200. For a 200-ppi scan, use a Radius setting of 1.0. For a 300-ppi scan, use a Radius setting of 1.5. This results in a halo of 1/50 of an inch—1/100 of an inch for each fringe. It's big enough to provide the sharpening effect, but not so big as to give the impression of artificiality that big halos produce. Overly large Radius settings are the prime culprits in producing the ugly oversharpened look that stands out so markedly, so watch out if you increase this setting much. The one situation where a larger Radius setting might be used is for large images that people will never look at up close—billboards and (some) posters. In this case, you may need a larger Radius to make the sharpness really pop at the intended viewing distance (a hundredth of an inch isn't much at 30 metres)
AMOUNT The Amount setting adjusts how intense the fringes are on each side of a tonal shift—how much the tonal differences are accentuated. Large values make for big fringes (large tonal differences where edges meet); small values make for less significant fringes. If you use the settings described for Radius, an Amount setting of 200 percent is a good choice, or at least a good starting point. If the Amount value is set too high, you can get an artificial look. It's not much of a problem if you keep the Radius setting low, but if you increase both Radius and Amount, images go weird very quickly. Also be aware that as you increase the Amount setting, you start to reach a point of diminishing returns. The fringes around big tonal shifts end up going all the way to white and to black (a problem in itself, because the white fringes in particular show up as noticeable artifacts), so increasing the amount has no further effect in those areas. Since those areas of large tonal shifts are the ones you most want to accentuate to give the effect of sharpness, and since you can't accentuate them beyond black and white, increasing the Amount setting beyond a certain point is fruitless. Large values in the Amount field can also accentuate problems with noise and mottling, but you can avoid these problems by using the Threshold value.
THRESHOLD The threshold value specifies how far apart adjacent tonal values have to be (on a scale of 0 to 255) before the filter does anything to them. If Threshold is set to three, for instance, and adjacent sample points are of values 122 and 124 (a difference of two), they're unaffected by sharpening. So low contrast areas— those with smooth gradations—aren't affected; the gradations stay smooth. The Threshold setting is the key to avoiding the mottling, speckling, and artifact-related problems that sharpening can cause. It causes the filter to ignore those slightly-out-of-place pixels, rather than accentuating them. Low Threshold values result in an overall sharper-looking image (because fewer areas are excluded). High values result in less sharpening. A setting of three or four for most images is recommended. NOTE: Settings of 10 and above aren't advisable, because they exclude so many areas as to reduce sharpening PHOTOSHOP: Unsharp Masking (USM) - 4
to near invisibility.
Effect of Photoshop's Unsharp Mask Filter The top scanned image has had no sharpening applied. The bottom image is the same scanned file which has had Photoshop's Unsharp Masking filter applied, to compensate for the softness introduced during the scanning process.
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Unsharp Mask Filter Settings The scanned file has a resolution of 300 ppi and is to be viewed at reading distance. RADIUS If we use the formula: image resolution รท 200 we can see that a Radius setting of 1.5 pixels should be used. AMOUNT Most images benefit with the amount set to around 200 %. THRESHOLD In most cases the Threshold is set to 3 or 4 levels.
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