Today’s Question: A friend of mine who is much more experienced in Photoshop than me suggested that I use a High Pass filter to sharpen my images. Can you tell me what that is, how it works, and how I should or shouldn’t incorporate it into my workflow.
Tim’s Quick Answer: The High Pass filter enables an effect that is very similar to what you can achieve with the Clarity slider in Adobe Camera Raw (and Lightroom). It is similar in concept to sharpening, but provides more of a local contrast enhancement that reduces the appearance of haze (and increases perceived detail and sharpness). The technique can certainly be helpful, though I find that the Clarity adjustment often provides a simpler solution.
More Detail: I think one of the best ways to get a sense for the High Pass sharpening technique is to actually try it out on a variety of different images. The process is rather straightforward, and can be automated with an action in Photoshop if you feel you’ll be using this technique on a regular basis.
The first step is to create a copy of the Background image layer, which you can do by dragging the thumbnail for the Background layer on the Layers panel to the “Create a New Layer” button (the blank sheet of paper icon) at the bottom of the Layers panel.
Next, change the blend mode for the new Background Copy layer to Overlay, from the default value of Normal, using the popup at the top-left of the Layers panel. This will create an effect of relatively high contrast in the image, which will be mitigated with the next step.
Now choose Filter > Other > High Pass from the menu to bring up the High Pass filter dialog. The High Pass filter creates an effect similar to an embossed look for the photo, which combines with the Overlay blend mode to enhance local contrast in the image. Start with a value of 10 pixels for the Amount slider, and adjust based on the effect in the image. Note that at this stage the effect will still likely be a bit strong. With practice you’ll get a sense of what value will work best based on the contents and resolution of the photo you’re working with.
Finally, reduce the Opacity setting for the Background Copy layer using the control at the top-right of the Layers panel. This allows you to mitigate the overall strength of the effect for the photo.
There is no question that the use of the High Pass filter in this way can help improve the overall perceived level of detail and sharpness in a photo. Again, the result is very similar to what you could otherwise achieve with a positive value for the Clarity adjustment available in Adobe Camera Raw or Lightroom’s Develop module. That said, for situations where you want to enhance the detail in a photo without the risk of creating the “crunchy” look that can result from excessive sharpening, this High Pass technique can work very well.