High Pass Sharpening – Tutorial

The typical last step in editing a photo is to apply sharpening. Sharpening is necessary because photos straight out of a digital camera or scanned film images tend to be a bit ‘soft’. That is, they don’t have the crispness that we may want.

The most common method of sharpening is to use the Unsharp Mask (Filter>Sharpen>Unsharp Mask). The USM filter will be familiar to those who’ve worked in a wet darkroom because it works the same way the unsharp mask worked in making wet prints. USM isn’t necessarily the best way to sharpen a digital photo; however. First, it’s a destructive method of sharpening. That is, it alters pixel values in the image file. This can be worked around if sharpening is applied on a separate layer but even then it’s less than perfect. Second, it can be a bit finicky to use at times. Getting just the right amount of sharpening can be difficult. It’s also easy to overdue the effect. Overcook the sharpening and you’ll end up with unpleasant bright halos around the edges of elements in the photo.

A more subtle, non-destructive and better overall method of sharpening is to use a High Pass Filter. The High Pass Filter works by detecting edges in a photo (in that respect, it’s not unlike the USM). Once you’ve applied the filter, you adjust the layer Blend Mode to give you the sharpening effect.
Here’s how it’s done.

Duplicate your Background layer (Layer>Duplicate). Then go to Filter>Other>High Pass. A dialogue box will come up on screen. You want to adjust the radius of the filter to a value of about 10. 10 is a good number for this effect. Anything lower and there isn’t enough edge detection for the sharpening to be evident. Higher than 10 and the area selected by the filter will be too wide which can give unpleasant results.

Click OK to apply the High Pass Filter. You’ll see a grey mask appear over your image file and the edges will be shown with less of a mask. This is normal. Now go to the Blending Options on the Layer Palette for the High Pass layer and select Soft Light. You should see the effect immediately. If you want to reduce the effect, lower the Opacity of the layer. If you want to enhance the effect, select Hard Light or Overlay from the Blending Options and then adjust the layer Opacity to suit.

Because you’re applying the sharpening on a separate layer, the effect is non-destructive. Saving the image file with the layers intact allows you to go back and adjust or cancel the sharpening altogether with your original image file untouched. Even applying USM on a separate layer, it can’t be undone or adjusted without deleting the layer and redoing the sharpening. High Pass is much easier to adjust if needed.

The shot on the left is a 100% crop of a photo with no sharpening applied. The shot on the right is after a High Pass sharpening with the Blend Mode set to Soft Light and the layer Opacity set to 75%.

High Pass Example (click for larger version)

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