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High Pass Filter for Sharpening

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By Chris Malinao

If you’re a photographer who aims for the best in retouching photographs, you want to master two main techniques: frequency separation and high pass filter—frequency separation for tone and details, as with portraits, and high pass filter for sharpening. The two important matters for the photographer are cleaning up a photograph while still preserving details and sharpening.

Go to Filter > Other > High Pass… to open the High Pass filter dialog and choose a radius value. For this image, I used 3.5 as radius value and I liked the sharpening it rendered. When I used 5.0 as radius value, I needed to reduce its opacity to 80 percent to get to where I want.

Go to Filter > Other > High Pass… to open the High Pass filter dialog and choose a radius value. For this image, I used 3.5 as radius value and I liked the sharpening it rendered. When I used 5.0 as radius value, I needed to reduce its opacity to 80 percent to get to where I want.

It is unfortunate that the two best photo-editing techniques in Photoshop are named like they were rocket science; frequency separation sounds like you’re beaming radio signals to a galaxy far, far away and high pass filter sounds like  coffee barista’s best-kept secret; and worse, the thing is hidden deep in the menu list under the obscure name “Other.” Lame.

But fear not, both techniques are not hard to understand and are worth mastering because they are extremely useful for the photographer. Frequency separation in Photoshop is making two layers available, one to edit and improve details (high frequency) and the other to take care of color and tone (low frequency). The resulting photograph will have an overall pleasing appearance because the process will clean up and smooth out blotchiness and uneven tones while still preserving details like skin pores. Frequency separation has been discussed in the previous article.

What about High Pass Filter? High pass filter is a sharpening technique in Photoshop and is even more straightforward. It is very easy to apply. We want to use it because: 1) It does an excellent job of bringing out detail; 2) We have full control of the sharpening process because we see it as it happens; 3) How and where sharpening is applied in the photo can also be adjusted after applying it; the process is non-destructive.

Here are the steps to sharpen with the High Pass filter:

  1. Make a copy (CTRL + J) of the image and stay on the top layer.
  2. Go to Filter > Other > High Pass to open the High Pass dialog box.
  3. In this dialog box, you will see a grey preview and you will be asked to enter a Radius value. For most photographs, this will be between 0.5 and 5.0 measured in pixels. High pass detects the edges and it’s asking you, “How many pixels from the edge do you want sharpened?” While at first a guessing game, with practice you gain confidence how much this will be for specific images. What you’re looking for is a good grey outline of the image with just the outline of shapes showing through. You want the details to appear, but the edges are still thin and the looks embossed. Enter the radius value and click Ok.
  4. The image on the top layer now looks grey. While still on this layer, change the Blend mode to Overlay. Click on its eye icon to toggle its visibility. If you like what you see, you can stop here.

If you see too much contrast, or you see “halos,” those white fringes on the edges—where dark pixels meet lighter pixels—the radius value you entered was too high. Go back and repeat the process and enter a lower radius value. If you only need to tone down what you see as over-sharpening, simply lower the Opacity value of the high pass layer; watch its real-time effect on the image as you reduce the opacity. Alternatively, you may simply add a layer mask and brush directly with black to hide the halos.

Instead of Overlay in Step 4, you may try the Soft Light blend mode; Soft Light is more subtle and has less contrast. Also, you may choose to sharpen only specific areas of your image by using a layer mask; with it, you can reduce or completely eliminate sharpening in other areas.

Regarding radius value in Step 3, images of people might require a smaller number, maybe between 0.5 to 2.5 but images with larger textures such as buildings may withstand higher values. In the example shown, I used 3.5 as radius value and I liked the sharpening it rendered. When I used 5.0 as radius value, I needed to reduce its opacity to 80 percent to get to where I want. I experimented with 20.0 and I saw much color fringing, so I first had to go to Image > Adjustments > Hue/Saturation and dialed down saturation to minus-100 (or zero) to remove color fringing before clicking Ok. The image had too much contrast so I had to tone down opacity to 60 percent to get to where I want.

By experimenting with different radius values, one gets to understand the effect of the High Pass filter. Don’t be afraid to experiment. The advantage of the High Pass filter is that you only have to contend with one number, its radius value. The High Pass filter is one great way to sharpen our photos. It gives us full control over the sharpening process and it does an excellent job of bringing out details in our photographs. Try it!

[Chris Malinao teaches digital post-processing methods such as the Lightroom workflow software and Photoshop editing software to photography students at the FPPF (Federation of Philippine Photographers Foundation), a non-profit organization that offers year-round workshops in Comprehensive Digital Photography, Lighting Essentials, Wedding Photography, Strobist Lighting, Food Photography, Photoshop, Lightroom, and other specialty photography workshops. For details of FPPF workshops, please visit www.photoworldmanila.com.]

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