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Unsharp Mask is a sharpening filter with a bit of an odd name, but with a lot of flexibility and utility. The name actually comes from a technique used in the wet darkroom where a slightly out of focus transparency was sandwiched with the original transparency to enhance sharpness in the print. In this lesson, we'll take a look at how you can use the Unsharp Mask filter to apply sharpening to your image. In this case, I've already prepared the image for its final output so I'm ready to apply sharpening.
I'll go ahead and choose Filter > Sharpen, and then Unsharp Mask from the menu in order to bring up the Unsharp Mask dialog. Notice that the preview in the Unsharp Mask dialog is set to 100% by default, which is perfect for evaluating the effect of sharpening. I've also resized my image to fit the available space, so that I can simply click in the image if I want to change which area of the image I'm evaluating while applying sharpening. Let's get atarted. You can see that I have an Amount slider, a Radius slider, and a Threshold slider.
Amount determines the intensity of the halos that we're adding along contrast edges in the image, the Radius adjustment allows us to determine the size of those sharpening halos. And Threshold allows us to mitigate sharpening in areas of relatively smooth texture. Quite often, I'll get started with Unsharp Mask by reducing the threshold value to its minimum of zero so that all areas of the image will receive sharpening, and increase the amount to its maximum value of 500%. This'll produce an exaggerated effect that helps me better evaluate the radius size.
For images with relatively high detail, I'll want to use a relatively low radius, generally around 1.0 pixels or below. For images where the detail is relatively subtle, where those transitions occur across a larger number of pixels, then I might need to use a larger radius setting. But even in those cases, I'll rarely go beyond about two or three pixels. Keep in mind also that these settings are relative to the size or resolution of the actual image. In other words, the more pixels there are, the more pixels we might need for that radius in order to be able to see the effect of sharpening. That means the guidelines for amount and radius are really just rules of thumb to help you get started.
You'll need to evaluate the image itself in order to determine the very best result. In this case, I have a relatively high degree of fine detail, and so I'll want a relatively low radius setting. I'll go ahead and reduce the value, paying attention to the size of the halos in my preview image. In any time, I can click on the Preview to see the original unsharpened image and I can release the mouse then to see the preview of the sharpening effect. In this case, I want the radius to be fairly small so I'll go reduce down below one.
And right about there looks to be a pretty good effect, somewhere around 0.8 or 0.9 seems to be a good effect overall for the image. Of course, with my amount up at 500%, I'm still applying much too aggressive a sharpening effect. So now, I'll go ahead and reduce that amount, again, paying attention to that preview image, trying to produce the very best result. I want a nice, crisp image without an over sharpening affect. I'll click on the Preview image again, to see the before. And you can see that I'm producing a rather strong sharpening effect, in fact, a little too strong.
I'll go ahead and reduce my amount a little bit more, and that looks to be much better. Now, in the process of applying this sharpening, I'm also effectively sharpening that background area. It's a relatively smooth texture. If I click on my Preview though, you'll see that it was smoother before I applied sharpening. I can prevent smooth areas from getting any sharpening effect through the use of Threshold control. In this case, of course, the effect is rather subtle so I'm going to exaggerate my sharpening effect just so that you can better see the sharpening in that background.
This will make it easier to see exactly what's going on with threshold. Now, keep in mind that sharpening is enhancing contrast, where contrast already exists. We can define contrast as the number of tonal values difference between two adjoining pixels. The higher the contrast, the bigger the difference between those pixels. The Threshold control allows us to specify that a higher degree of contrast must exist before sharpening is applied. So, for example, if I increase the Threshold value, you'll see that the background area, retains its smoothness.
However, the bird which contains at high degree of fine detail is still receiving a strong amount of sharpening. Now, I don't want to use too much threshold, generally speaking, only about 4 to 8 levels will be necessary in order to retain smooth texture in areas of the image. So, I'll use the minimum amount need in order to create that effect in order to preserve smooth textures in areas where it's important. Now, in this case, with an exaggerated effect, I need to go a little bit higher than I otherwise would, but I think you get the idea here.
I want to use the minimum threshold value needed in order to produce the desired result. In this case, of course, I have exaggerating sharpening applied. So, while I've set the threshold to a desirable value, I do need to go back and fine tune my overall sharpening effect. And there we have it, a good effect that's sharpening the overall image, the most important areas of the image, without applying sharpening to that smooth background area. The Unsharp Mask feature is certainly not the most sophisticated sharpening filter, but it does over a great solution for basic sharpening of your photos.
In particular, the Rhreshold setting makes Unchart Mask valuable for a wide variety of images.
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