Most images can benefit from sharpening, which enhances the edges by increasing the contrast of adjacent pixels. Photoshop includes several filters that sharpen areas of an image. Which filter you use depends on whether you need to sharpen all or only part of an image, and how much control you require. No matter which method you use to sharpen an image, be careful not to oversharpen. Sharpening brings out the detail in images, but it can't compensate for severely blurred images.
Here are some tips for sharpening images:
- Always work with a duplicate of the image, so you can return to the original if you aren't wild about the changes you've made. Additionally, you can convert layers to Smart Objects and then use Smart Filters to sharpen them.
- Create a duplicate layer and apply sharpening to that layer, so you can turn it off if you need to sharpen differently later. If you work with a duplicate layer, use the Luminosity blending mode for the best results.
- Save sharpening for the last step, when you're just about to optimize your image for the Web or prepare to print it. The exception is if you need to sharpen to correct some blur caused by the camera or scanner; that sharpening should occur at the beginning of your workflow.
- The Sharpen, Sharpen Edges, and Sharpen More filters are automatic. The Smart Sharpen and Unsharp Mask filters give you more control.
The Smart Sharpen filter performs multiple tasks at once. In addition to sharpening edges, it reduces noise. You can choose whether to apply the filter to the entire image, its shadows, or its highlights.
You can also choose which algorithm the filter uses to sharpen the image:
- Gaussian Blur uses the same algorithm as the Unsharp Mask filter; it increases the contrast along the edges.
- Lens Blur detects edges and detail; it sharpens finer detail without adding many halos.
- Motion Blur removes blur that was caused by the movement of either the camera or the subject of the photo. If you choose Motion Blur, you also need to specify the angle for the blur.
Figure 4.25 The Smart Sharpen filter gives you control over the way Photoshop sharpens your image.
To use the Smart Sharpen filter:
- Zoom to 100 percent or greater so you'll see an accurate preview of the sharpening.
- Choose Filter > Sharpen > Smart Sharpen.
- Select an option from the Remove menu to determine which algorithm is used to sharpen the image.
- Keep an eye on the preview window as you adjust the Amount and Radius sliders. The Amount slider determines how much sharpening occurs; the higher the amount, the greater the contrast between edge pixels. The Radius slider determines how many pixels surrounding the edge pixels are affected by sharpening.
- Select More Accurate if you want Photoshop to process the file more slowly, resulting in a more accurate sharpening.
- If you want to adjust the sharpening of shadows and highlights separately, click the Advanced button to display additional tabs. The settings on the Shadow and Highlight tabs let you reduce halo artifacts.
- Click OK when you're satisfied with the sharpening.
It sounds like it ought to blur the image, or make it "unsharp," but the Unsharp Mask filter actually sharpens images. Its name comes from a traditional darkroom technique, in which the original blurry negative of the image was overlaid with an attempt at sharpened edges, and a final print was created from that.
The Unsharp Mask filter removes Gaussian blur by increasing the contrast along the edges: it makes the dark areas darker and the light areas lighter. The filter determines which pixels to darken or lighten based on the pixels near them. The radius determines how large an area the filter compares. If you go too far with sharpening, you'll probably see a halo effect around the edges.
Figure 4.26 Use the Unsharp Mask filter to sharpen the edges in an image.
To use the Unsharp Mask filter:
- Choose Filter > Sharpen > Unsharp Mask.
- Select the Preview option if it's not already selected. When Preview is selected, you can preview the filter results in the full image window. Just deselect Preview to compare the sharpened image with the original.
- Set the Radius value. Unless you're going for a dramatic, artistic effect, keep the radius pretty low, usually between 1 and 2 pixels.
- Adjust the Amount slider to increase or decrease the contrast. For high-resolution printing, Adobe recommends 150 percent to 200 percent.
- Set the Threshold value. The threshold determines how different the pixels must be from the surrounding area to be considered edge pixels. The default threshold, 0, sharpens every pixel in the image. For most images, you'll do best with a threshold between 2 and 20.
- Continue adjusting the sliders until you reach a combination that works for your image, and then click OK to make the change.