How to Use the High Pass Filter to Sharpen Images

Photo editing programs offer various methods for you to sharpen your photos and the High Pass filter is one of the easiest yet most effective tools for the job. It selects the edges found in the image and sharpens them without affecting the areas that have no edges.

People have personal preferences over which sharpening tool works best for them and the High Pass filter is arguably the most popular for good reasons. One is that it locates the edges for you and you don’t have to manually select areas as you would with the Sharpen filter or Unsharp Mask (insert link to unsharp mask article). Another reason is that adjusting the filter is very simple since there are less settings to tweak compared to the others; in fact there is only one (unlike the Unsharp Mask which has three settings and the Smart Sharpen which has even more.)

Here is a short tutorial on how to use the High Pass filter. Adobe Photoshop CS2 is used for this purpose and you might be using another photo editing program, but the concept works the same. If it is your first time ever to use this technique, you would be surprised by how quickly you will get the hang of it and make it a part of your post processing routine.

Step 1:

Open a copy of your original image (one can’t be too careful with keeping the original image safe!) and create a duplicate layer. The keyboard shortcut is Ctrl + J for Windows and Command + J for Mac.

Step 2:

Change the duplicate layer’s blend mode from ‘Normal’ to ‘Overlay.’ You can find the blend mode options in the Layers Styles dialog box at the bottom right of the screen. The High Pass Filter works by turning to neutral gray all the areas of the image that do not have edges and the Overlay blend mode keeps all neutral gray areas from being adjusted. Therefore, the neutral gray areas will not be included in the sharpening.

When you click on the Overlay blend mode, your image will suddenly appear to have more contrast like this:

This saturated effect will disappear once we click on the High Pass filter.

Step 3:

While still highlighting the duplicate layer, click on the Filter menu (found on the toolbar on top of the screen) and choose ‘Other’ from the dropdown list, and then click on ‘High Pass.’ The High Pass dialog box will appear showing the preview of an area of the image in gray (edges are darker gray), the preview option and the one and only setting we mentioned earlier, the Radius setting.

The Radius slider controls the amount of sharpening to be applied around the edges. Dragging the slider to the right will increase the radius value (in pixels) and affect a bigger area, while sliding it to the left reduces the intensity of the effect. By keeping the preview checkbox ticked, you can see how the image in the main screen is affected when you move the radius left or right.

How much radius value to use depends on the pixel dimension of your image. Generally, the larger the image size, the higher the radius value you’ll need to use. A sign that you’re going overboard with the radius value is when halo effects begin to show up around the edges of details. This image size is rather small so a radius value of 1 pixel is enough to sharpen the edges without causing ill effects from oversharpening.

Once you have chosen the radius value, just press ‘OK’ and you’re all done!

The photos below show the difference between the unedited image (photo on left) and the result of using a High Pass filter (photo on right):

Notice that the areas with edges, especially the hair, eyes, lips and the yellow curls, have become more defined while the areas with no edges, namely the cheeks, have remained smooth.