This is the bottom left of the "B" in "Believe". In the original the transition between blue and gray wasn't completely sharp. It shades from gray to blue over a couple of pixels. Dan says this is caused by, "the real life line of transition being narrower than ... even ... film ... can resolve." The after image shows clearly how the USM magic trick works. The dark blue area has been surrounded with a light colored halo in the gray area. And the lighter gray area has been surrounded with a darker colored halo in the blue area.
What sharpening can't do
Understanding how sharpening works leads to an understanding of its limitations. When I first heard about it, I thought, "Just what I need, a way to correct fuzzy out-of-focus shots." But sharpening cannot help where transitions aren't fairly crisp. It works by looking for transitions finner than some threshold (more about this soon.) If there are now such transitions, it does nothing and thus has no effect. Sharpening also can't help images without sharp transitions. Skin for example, is usually lacking in such transitions, and the ones that it does have are things we don't want to emphasize (wrinkles, pimples, etc.) On the other hand, portraits usually have things we do want to sharpen, eyes, hair, clothing, and things we really don't, skin for example. This is often true and a large part of the second tutorial is devoted to fine tuning so that sharpening does what we want and doesn't do what we don't want.
When to sharpen
Sharpen last after, any color correction, cropping, black and white conversion, not to mention composting and edits involving masks, brushes or cloning. Steepen curves after sharpening and you effectively change the amount parameter with unpredictable results. Sharpen before masking, extraction, composting, or cloning and you make you job all the harder and will likely end up with unnatural looking results. Once you become proficient at sharpening in post processing, you will want to disable in camera sharpening because you will want to sharpen yourself after other edits. Sharpening twice is generally a bad idea. Users of raw conversion software also will want to disable sharpening during conversion. Users of ACR should disable such automatic sharpening by following the arrow to the right of "Settings Selected Image" to the preferences menu. Option "Apply sharpening to preview images only".
Prepress professionals preparing images for publication sharpen with knowledge of the actual size of the reproduction, but that's probably to much to ask under most circumstances. If you are very prefectionistic, though, and want huge prints, it is a good idea to sharpen separately for them.
Unsharp mask recipe
The examples I gave above illustrate one of the frustrating things about sharpening. There is no pat formula that you can apply to all your photographs; each image requires some work to determine the correct sharpening parameters. It's actually worse than that. The correct sharpening parameters are also a function of the size the image will be reproduced. Large prints require a very light hand with the sharpening parameters. Images for posting on the Internet may require quite a bit of sharpening before the effect is noticeable and in many cases this poses insurmountable problems. The amount of sharpening required to make a visible difference is often so much that at least some parts of the image are over sharpened. Thus I usually take the approach of sharpening for largish sized prints and letting it go at that. In the cases where it matters most, this helps images posted on the web. It never make a mess. And it makes prints look great. The demolition derby and rooster pictures illustrate this. At dgrin L size, the effect of sharpening is subtle. In prints, it would be dramatic. The "Believe" picture is over sharpened for the sake of illustration. Here the difference is dramatic, even at this small size. A print of the sharpened version has visible halos that are very unattractive and distracting.
Take heart, though, there is a simple 9 step procedure that produces good results for many shots. I'll outline the steps first and go through in detail with illustrations.
The Simple 9 Step Procedure
- Work in the LAB color space. If the image is not already in LAB mode, use Image->Mode->LAB to get it there. Select the L channel by clicking on it and then click the box to the left of the composite LAB channel in order to make all the channels visible at once.
- Work with 100% magnification. Select some important part of your image, eyes and hair for example. If you care more about how your picture will look posted on the web, work at lower magnification. If you care more about very large prints work at higher magnification. Dan taught me that 100% was a good compromise, and I have found this to be true.
- Bring up the USM dialog box with Filters->Sharpen->Unsharp Mask.
- Set the "Amount" slider to it's highest setting, 500.
- ]Set the "Radius" slider to 5. The slider actually goes much higher, but trust me, this is a very high setting.
- Tune the "Threshold" slider so that noise isn't being sharpened. With the slider at 0, you will see lots of ugly noise in areas that should be solid. Increase the threshold until only features that you actually want sharpened are affected. I often find that values between 10 and 30 work well. (Use the "preview" check box in this step and future steps to compare the unsharpened image with your current parameters.)
- Tune the "Radius" parameter so that the halos are not so large as to obscure fine detail. Large halos can extend into neighboring areas and make a real mess. The correct values for this parameter are very dependent on the resolution of the image. For 8MP images, I find the right value is often somewhere between 1 and 3.
- Tune the "Amount" parameter until the image actually looks good at 100%. This means turning it down until the halos are not quite visible but their effect is. Use the "preview" check box often here to compare with the original. The sharpened image should look sharper, but the halos should not be obvious. You may need to iterate a few times between steps 7. and 8. to fine tune a bit more.
- Apply the filter and zoom to fit the image to your screen. Use undo/redo to compare the image before and after.
Now, I'll walk though the steps in more detail with illustrations.
photoshop tutorialusm sharpenphotoshoptutorialusmsharpenunsharpmaskunsharp masksharpening