Sharpening Simplified; The High Pass Technique

No matter how careful you are, soft happens. Your subject, whether a femme fatale or a flower, may move. The light level may be low so you have to shoot at a slow shutter speed, and camera movement blurs the photo slightly. Or you may be photographing under bright overcast conditions outdoors--plenty of light for most situations--but you're shooting soccer, skating, baseball, or some other fast paced sport. With a long lens, you set a high shutter speed to stop the action and prevent lens blur, but sometimes the best frame is slightly unsharp.

Unsharpness isn't limited to shooting scenarios. You can get it on your digital desktop, as well. Whenever you scan film or a print, some softness is introduced. In Photoshop or Elements, if you use the Transform tool to correct perspective or otherwise alter your photo, some softening is likely to appear in the transformed areas. What remedies do you have?

While Adobe Photoshop has a plethora of sharpening choices under the Filter>Sharpen menu, there are only two that most pros rely on--Unsharp Mask and (in CS2 and above) Smart Sharpen. Both of these can require some experimentation, time, expertise, and patience. High Pass sharpening, on the other hand, is available in earlier versions of Photoshop as well, and can be accomplished fairly quickly and easily. It is a "secret" technique used by professional retouchers to accomplish fine sharpening and is especially effective with images that contain large areas of smooth tones such as skin, sky and building surfaces. It can work well with images that have lots of edges, such as architecture or automobiles.

Two features distinguish High Pass sharpening. First, it does not create as many artifacts as other sharpening methods. These manifest themselves as an overall grittiness, especially visible in large areas of smooth tones. This grainy effect is common when Unsharp Mask sharpening is cranked up. Secondly, the High Pass approach is totally non-destructive and doesn't change an iota of your original image pixels. It works by identifying edges within your photo and then increasing the contrast between the sides of the edge. More pronounced edges translate into greater perceived sharpness.

Fine Tuning Controls
You have several options to fine-tune the sharpening effect. First, you can see how choosing a different radius setting in the High Pass dialog box affects your image. Second, you can change the Blend mode. Soft light yields a subtle effect. The sharpening (and contrast) become progressively more pronounced when you shift to these Blend modes: Overlay, then Hard Light, then Vivid Light. To toggle between before and after views, simply click on the eye icon of the copy layer in the Layers palette.

A third way to vary the sharpening effect is to lessen it by lowering the opacity slider amount for the duplicate layer. You'll find the opacity slider at the upper right of the Layers palette. Finally, a trick some professional retouchers use is to apply a small amount of High Pass sharpening--say, a radius of 3 or less--and then duplicate the High Pass layer one or more times.

High Pass Sharpening, Step By Step
1. Open your original. This shot of Sybille with orange flowers is about 8x10" at 300dpi, file size about 20MB.
2. In the Layers palette, make a duplicate of your background layer. Do this by dragging the background layer to the New Layer icon at the bottom of the Layers palette, just to the left of the trash can icon.
3. In order to see the effect of the High Pass filter, change the Blending mode (circled in red) for the copy layer to Overlay, as shown. In this Blend mode, you'll be able to see the effect of different settings of the High Pass filter in real time as you apply them. Ignore the over saturated contrast and color, as you will change these with the next step.
4. From the menu bar, choose Filter>Other>High Pass.
5. This brings up the High Pass filter dialog box. For a file this size (8x10 at 300dpi, 20MB), try a radius setting of 10 pixels. For a smaller file, try a lower amount. What setting works best will depend on the type of image and how soft it is. I recommend that you experiment with a wide range of settings to see the variety of effects available.
6. If you forget to change the Blend mode to Overlay (step 3), this is what the High Pass copy layer will look like--an overall medium gray with the edges of the subjects in the photo outlined. If your image looks like this, simply change the Blend mode to Overlay.
7. Here is a detail of the original, slightly soft, photo. Look especially at the hair, glasses, and teeth.
8. With the High Pass filter radius set to 10 pixels and the Blend mode set to Overlay, note how the hair and glasses have been sharpened, and how the contrast and color saturation have been increased. Furthermore, the edge contrast of the green leaves and orange flowers has been pumped up considerably.