Unsharp Mask


Understanding USM. OK, now that I’ve colors about right, now its time to futz with the actual images in Photoshop. Some of the things are understandable. Changing the Brightness (e.g., the Black Level), so if a picture is very bright (has a high black level), you turn it down, this means that everything looks darker.
Contrast is the other common setting, this is actually the White Level of the photo, it means that if you turn it down, the difference between the dark and light goes away. Things get murkier.
The strangest thing to do though is Unsharp Mask. What a strange name. What it means is that technically another picture is created (the mask), this mask is then blurred (unsharpened), then the computer compares the original to the blurred to try to find edges. If it finds an edge of an object then it increases contrast. It makes the darker side darker and the light side lighter, so every thing looks better.
There are three parameters:
# Amount. This just means the amount by which you make the darker side darker and the lighter side lighter. I think of this as the “contrast” setting for the edges.
# Radius. This sets how much blurring is done, that is how unsharp the mask is. The higher the radius in pixels, the larger the area that is sharpened. That is, it makes the edges huge if you set it high, like a big crayola marker in your photo at high values.
# Threshold. This tells says how much of a difference between the blurred and the original copy are needed before you detect an edge. A higher number means, it has to be a very obvious edge. A low number means that it finds edges in just about everything.
“Unsharp Mask Settings”:http://www.scantips.com/simple6.html. Well, that’s great, but what are some parameters to use for typical pictures. There is lots of folklore, but scantips.com has some good rules of thumb:
# Radius controls how wide the edge rims become, and Radius = 1.0 is about the right ballpark, with 0.6 to 2.0 often being useful. Higher Radius values can cause halos at the edges, a detectable faint light rim around objects. Radius units are not the same as pixels, the units step in tenths, but the Radius width is usually at least 4 pixels overall, you will see various effects. Radius is a very important parameter, and the easiest way to ruin a good scan is with too much Radius. Inanimate objects can use the most radius, human faces can tolerate the least, and landscapes fall in between. But it really depends on the size of the details. Fine detail needs a smaller Radius, or else you may obliterate tiny detail of the same size as the Radius width. Large images have larger detail (more pixels involved) and can use more Radius, so therefore printing at higher resolution can support the larger radius. Radius and Amount interact, reducing one allows more of the other.
# Threshold specifies how far apart adjacent tonal values have to be (values of 0..255) before the filter does anything to the edges, before it is judged to be an edge at all. This lack of action is important to prevent smooth areas from becoming speckled. Low values should sharpen more because fewer areas are excluded. Higher threshold values exclude areas of lower contrast. Human faces want values greater than 1 or 2, like perhaps 5 or more. For inanimate objects, perhaps 0 or 1 is useful. General work, try 3 or 4. This control has little effect at high values, but has more effect changing between low values of 0 to 5. This Threshold is not to be confused with Line art Threshold.
# Amount is like a volume control, exaggerating the edge differences (how much darker and how much lighter the edge borders become). Amount interacts with Radius as to degree of sharpening, but it does not affect the width of the edge rims. Amount has a large effect, and values of 80 to 120 are normally usable if the Radius isn’t too large.
“Aaraich.com on what to do if you don’t know”:http://www.arraich.com/ps_tips_shrpn5.htm. A basic, simple procedure to use, when you want to apply sharpening with the Unsharp Mask, but don?t know what settings to use, is:
# Set the Amount slider to the maximum 500 %.
# Increase the Threshold slider until sharpening in unwanted areas such as skin, and shadows has disappeared.
# Increase the Radius setting as much as you can without obliterating needed details. This can vary quite a lot from image to image.
# Go back to the Amount slider, and set it for as much or as little as you like, which will most frequently be somewhere between 100 and 200 %.
# If you have problems with color halos appearing on your sharpened edges, a commonly used technique is to choose Filter > Fade immediately after applying the sharpening. In the Fade dialog box that appears, change the blend mode to Luminosity, and possibly lower the Opacity setting. For Photoshop v. 6.0 users, the Fade command is under Edit > Fade
# As a last note, don’t sharpen too much, it makes everything look like an alien planet if you are not careful.
“Earthboundlight”:http://www.earthboundlight.com/phototips/unsharp-mask.html. has another suggestion for how to use this…
# View. Set it to 100% view so you can really see what is happening to the image.
# Radius. An image with fine detail will need a lower Radius setting while one with less inherent detail can utilize a larger Radius. Start with a Radius between about 1.0 and 1.5 and go from there, lowering the setting perhaps to 0.5 or even lower for images that start with fine details, or raising it to perhaps 2.0 to 4.0 for images with lower levels of inherent detail.
# Amount. If you have selected a small Radius setting, the halos will be narrow, so you will need a high Amount setting for them to have much effect. Conversely, if you are using a high Radius value, the halos will be wide and you will need a low Amount setting to avoid them being obvious. Once you understand what the controls actually do, you should be able to tweak things visually using the 100% view. Play with a few of your own images until you get the hang of things.
# Threshold. After settling on ranges for Radius and Amount, increase the Threshold value until unwanted noise or other artifacts start appearing where you don’t think edges should be, then lower it slightly.
Finally, “About”:http://graphicssoft.about.com/od/photoshoptutorialssharpen/ has collected a bunch of tutorials that all contradict each other, so go figure ;0)

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