Digital Darkroom

In my last column I covered using Unsharp Mask to sharpen your images for output, whether print or screen. This time, we'll go a step further and talk about selective sharpening. Why would you want to use selective sharpening? It's the ideal choice when working on portraits, where you want to keep the skin smooth but have good detail and focus on the eyes. You'll also find selective sharpening useful when you have soft blurry backgrounds with an in-focus subject like a flower. Using Unsharp Mask on these image types can render the backgrounds too harsh, detracting from the main subject.

To get started, we'll duplicate our image onto a new layer in Photoshop (any image-editing program that supports layers and masking can handle this task, but I'm using Photoshop CS2 for these examples). If you've been working on your image and have multiple adjustment layers already, you can flatten those adjustments into a new layer to begin sharpening by using Alt + Control + Shift + E to merge all visible layers into a new layer.

Using Merge Visible Layers, you can retain all of your adjustments without flattening the image. This is helpful because sharpening should be the last step in your workflow.

On this new layer, apply Unsharp Mask to the entire layer using the guidelines presented in my last column (October 2006 issue of Shutterbug, or on the web at www.shutterbug.com). Remember that Unsharp Mask works by enhancing the contrast between edges in your image. The Amount adjustment controls how strong the contrast will be; Radius controls how wide the area considered an edge is; and Threshold sets the limit for how different adjacent pixels must be to be considered an edge.

At this time, you need only worry about how the primary subject, or the area you want sharp, looks. Don't worry about the other areas of your image, we're going to fix those soon. If you're doing a portrait, sharpen for the best possible detail in the subject's eyes. For other types of photos, sharpen for the main point of interest.

For this image, I'm sharpening to get the most detail from the raindrops along the edge of the flower and the stem. These settings will render the blurred background a bit too harsh though. Layer masks to the rescue!

Now, we're going to add a layer mask to the image to hide the sharpening from the areas we don't want. A layer mask works by painting over your image with black or white. The simple way to remember which is which--black conceals, white reveals. By painting over the image with the Paintbrush tool using black, you hide the sharpening adjustments for that part of the image, leaving only those parts you want sharpened. Choose Layers>Layer Mask>Hide All (I find this to be faster when most of the image will not be sharpened) and you'll see that your changes have all seemingly disappeared, and the icon in the Layers palette will be filled with black. Alternatively, you can click on the Layer Mask icon in the Layers palette which will give you a new mask with all areas visible.

After applying the Unsharp Mask filter, create a layer mask and fill with black to hide the adjustments. From here, I paint using white to reveal the sharpened image below.

If your program doesn't support layer masks, an alternative to this method is to erase the sharpened layer where you don't want the effect to be visible. This isn't as effective though because you have less control over the effect.