When you’re working on your digital images with Photoshop you might face this situation: you want to emphasize one particular area of the photograph by adding a bit of contrast and sharpening to that area, while not affecting the rest of the image. You could make a selection and run a Curves adjustment layer on the selection to add contrast, but how can you sharpen a local area? Here’s a way to use the High Pass filter to solve this problem.

I’ll use this image from Petrified Forest National Park in northern Arizona, taken on a day with thick overcast. This was captured as a RAW file with my D2x Nikon. I opened it in Photoshop CS2 and made some small tweaks in adjustment layers.

Fine…but now what I want to do is to emphasize the foreground rock just a bit more.

I want to merge all the layers into a new layer, but without losing any of the workflow steps I have made. To create this new layer, I held down the Alt key, then clicked on the right-facing little triangle at the top right of the layers palatte and then selected “Merge Visible Layers” from the fly-out menu. A keyboard shortcut for doing this is Alt-Ctrl-shift-N-E. (If you can call that a shortcut!) Here is the Layers palette showing the new merged Layer 1.

With this new merged layer active I select Filter > Other > High Pass, and use a setting between 30 and 50 pixels. I’ll use 50 in this example just to make the effect more visible.

My screen image becomes really weird looking, but not to worry.

The Layers palette shows what the High Pass filter has done as the small layer thumbnail shows the weird coloration.

Now I change the layer blending mode to soft light. This makes the image colors switch back to normal.

The entire screen image now looks over-sharpened. I hold down the Alt key and click on the Add a Layer Mask icon at the bottom of the Layers palette; this adds a black layer mask which blocks the High Pass filter’s effect from showing. I could have selected Layer > Layer Mask > Hide All to get to the same point.

Now the Layers palette shows the black layer mask.

I select the paintbrush tool and check that white is the foreground color on the tools palette (the D key gives the default black and white colors while the X key exchanges the foreground/background colors). I paint over the rock and the black layer mask thumbnail reflects where I paint.

I drop the layer opacity until I like the effect.

Here’s the final image.

The first image below is a tight crop of the rock before I used the High Pass filter while the image below it is the same section after the High Pass filter, but before I lessened the effect by lowering the layer opacity.

	John Shaw

This is not a technique you’ll use every day, but it is certainly useful when you want to emphasize one small area. While I’ve exaggerated the settings here for illustration purposes, you should be careful not to overdo the effect.

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