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This chapter is from the book

This chapter is from the book

The Unsharp Mask Filter

Step 17: If you are going to resample your image by either adding pixels to make it bigger or removing pixels to make it smaller, you should do that first before sharpening your file. In the Image/Image Size dialog, if you are just changing the Pixels/Inch with the Resample Image option turned off, this should not affect the sharpening of an already sharpened file as you are not adding or removing pixels.

Choose Image/Duplicate (F5) to make a copy of this master layers file called KansasVersion1, turning on the Duplicate Merged Layers Only option and then choosing OK in the Duplicate dialog. Type an F to put this single layer copy in Full Screen mode. Now you will use Filter/Sharpen/Unsharp Mask to sharpen your image for final output. The Unsharp Mask filter has three different settings (Amount, Radius, and Threshold) that affect different parts of the sharpening process. You will have to run some tests to determine what value to use in each of these settings. It is often useful to compare tests on a small section of the image. Photoshop does have a Preview button in the Unsharp Mask filter that allows you to see the effect on a selected area of the image, but it doesn't allow you to compare one group of settings to another. Once you get familiar with the amount of sharpening you like on your various types of prints, you can turn the Preview button in the Unsharp Mask dialog off and on while trying different settings to decide the amount of sharpening you want. Turning the Preview button off and on will change the part of the image that is visible on your computer screen from not sharpened to sharpened. The small window inside the Unsharp Mask filter will always show part of the image as sharpened, but if you click down and hold inside that little window, that part of the image will toggle to unsharpened. Click anywhere in your image to reset what actually shows within this window inside the Unsharp Mask filter.

Figure 20.32Figure 20.32 Step 17: The Unsharp Mask filter dialog box with the settings I used to sharpen my KansasVersion1 file.


Another technique for sharpening very saturated files is using Image/Mode/Lab Color to convert the file to Lab Color mode and then sharpening the L channel. This method prevents your saturated colors from popping too much during the sharpening. You can actually print to an Epson printer directly from Lab color so you don't need to convert back to RGB. Another sharpening option is to just sharpen one of the RGB channels.

A good way to compare different sharpening settings is to use the Marquee (M) to select a section of the image that can represent the entire image, and whose sharpness is most important, and make a copy of it using Edit/Copy. (See the images on the next page to see what I selected from this photograph.) Now choose File/New (Command-N) to create a new file. Because you just made a copy, the new file will be the size of the copied section. Click OK in the New dialog box and then do Edit/Paste (Command-V). Repeat this action several times, until you have five or six small files that you can place next to each other on the screen for comparison. Now run different tests on each file to see what each of the three parameters of Unsharp Mask do. Speaking of those three parameters, here is what they do:

Amount: This setting controls the overall amount of sharpening. When you compare sharpening effects, you want to zoom in on the image (to 100% and occasionally to 200%) to see all the detail. Compare different copies of the same image area using different settings for Amount. You sharpen an image by looking for edges in the photograph and enhancing those edges by making one side of them darker and the other side of them lighter. Edges are sharp color or contrast changes in an image.

Radius: This setting controls the number of pixels along an edge that you modify when you sharpen the image. Again, try running the filter with different settings and compare several copies of the same image side by side.

Threshold: When you set Threshold to 0, everything in the image becomes a candidate for being an edge and getting sharpened. If you set the Threshold to, say 10, then an edge will only be found and sharpened if there is a difference of at least 10 points (in the range from 0 to 255) in the pixel values along that edge. The larger the value you give to the Threshold setting, the more contrasty an edge needs to be before it is sharpened.

When you find the Unsharp Mask values that look best, use those to sharpen the entire file. If the original image is very grainy, I might increase Threshold, which lessens the sharpening of the grain. If the image is very fine grained, I might decrease Threshold, which allows me to sharpen the file a bit more, without getting more than the normal grain appearance in the final image. You have to be careful not to over-sharpen. If your final output is a halftone, you can get away with more sharpening than you can for a transparency film recorder, or even a digital print output, because the screen angles and dots in a halftone tend to lessen some sharpening artifacts. All artifacts show up if you output to a color transparency film recorder, however. We usually use the Unsharp Mask filter instead of the other Photoshop sharpening filters because Unsharp Mask provides much finer control over the many different types of images.

Another way to sharpen your image and avoid sky grain, which I usually use, is the Sharpen Only Edges BH action script. This is explained in Chapter 11: "Automating with Actions," and also at the end of Chapter 26. You should check it out, as it produces very sharp files without the grain!

Figure 20.33Figure 20.33 Step 17: If you name the layer in your document with the sharpening value you used, that information will help later if you decide the sharpening amount wasn't correct.

Figure 20.34Figure 20.34 Step 17: Image with no sharpening. We need some!

Figure 20.35Figure 20.35 Step 17: Unsharp Mask 150, 1.5, 0. Too much grain!

Figure 20.36Figure 20.36 Step 17: Unsharp Mask 450, 1.5, 0. Too much sharpening and grain!

Figure 20.37Figure 20.37 Step 17: Unsharp Mask 150, 4.5, 0. Too large a Radius for a real look.

Figure 20.38Figure 20.38 Step 17: Unsharp Mask 150, 1.5, 8. I used to use this a lot but now I experiment more on each image.

Figure 20.39Figure 20.39 Step 17: Unsharp Mask 300, .5, 4. Compare this to the ones on either side.

Figure 20.40Figure 20.40 Step 17: Unsharp Mask 500, .5, 8. Let's see how this prints.

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