Adobe Photoshop CS2/CS3/CS4/CS5
Macintosh, Microsoft Windows
While it’s frustrating to discover that an image is out of focus, it’s even more annoying trying to figure out the best sharpening technique. Don’t overlook an oldie but goodie—Photoshop’s Unsharp Mask filter—for bringing your soft images back into focus.
To sharpen with Unsharp Mask, we’ll:
• Take a quick look at where the term Unsharp Mask comes from.
• Examine the filter’s control options so you understand how to adjust the settings for your images.
• Selectively control the sharpening through the use of channels.
Although much depends on the quality of the image itself, there are a number of techniques you can use to increase image detail and crispness. A terrific aid to fix sharpening problems is Photoshop’s Unsharp Mask filter. Though you can’t use it to fix significantly blurry images, you can apply it to reasonably decent images and make them much sharper, as shown in Figure A .
The significance of the filter’s name
If you’re wondering why we use a filter named Unsharp Mask to sharpen an image, you aren’t alone. The term comes from the traditional film darkroom era. To sharpen a photograph, a photo technician first made a contact-masking negative of the original photo. Then, to reduce the harshness of the mask, he placed a thin sheet of frosted Mylar between the original and the mask. This softened the mask and made the image appear sharper.
Open a soft image example
You can use the Unsharp Mask on an image of any size or color mode. To follow along using our example, download the file unsharp.zip file from the URL given at the beginning of this article, extract the file unsharp.jpg , launch Photoshop, and open the file shown in Figure B . (Image provided by Jim Whitcomb.)
Our photo was taken on a foggy morning, and though its focus appears to be soft, it isn’t. There’s plenty of detail we can sharpen in the leaves and branches. Now, choose Filter > Sharpen > Unsharp Mask to display the Unsharp Mask dialog box, as shown in Figure C .
You’ll see that the dialog box contains a preview window and three different control sliders—Amount, Radius, and Threshold. Let’s look at the purpose of each control.
The Amount slider control affects the overall sharpness of your image. The greater the percentage value, the greater the contrast between pixels. For high-resolution printed images, you should stay within a suggested range of 130 to 200 percent. If you’re sharpening for the web, adjust the image until it looks good onscreen.
The Radius slider control determines how many pixels surrounding the edges within your image you affect with the filter. A lower value sharpens just the edge pixels and a higher value sharpens a wider band. For high-resolution images, we recommend a Radius value between 1 and 2.
Note: If you’re going to output your image to a printer, we suggest you determine the Radius value by dividing the printer’s output resolution by 200. Then, enter that number in the Radius text box. Be careful, however, for if you set the Radius too high, your image may actually lose detail and look as if it’s overexposed.
The Threshold slider controls the numerical difference the sharpened pixels have to be from the surrounding pixels before they’re sharpened as well. For example, a Threshold level of 10 won’t affect any pixel color value that’s within ten level values of the edge pixels. Use a value between 3 and 10 as a starting point. By keeping the Threshold value low, you’ll avoid sharpening areas or adding excess noise to your images that don’t need it, such as skin or deep shadow areas.
Apply the Unsharp Mask filter
Now that you know the purpose of the Unsharp Mask filter control options, let’s apply the filter. Enter in the Amount, Radius, and Threshold text boxes values of 75, 2.0, and 5 respectively.
Make sure you select the Preview check box so you can see how the filter affects your image. By comparing before and after areas in our photo, you’ll note the slight image sharpening, as shown in Figure D . It’s a good start, but not quite what we’re after. Let’s adjust each of the sliders one at a time and note the sharpening effect on our image.
To do so, follow these steps:
1. Drag the Amount slider all the way to the right. The result overcompensates, as shown in Figure E , so move it back to the left for a more pleasing result at a value of about 140%, for results shown in Figure F .