High pass filter sharpening is a way to sharpen images without the jagged artifacts that the sharpening tools in many programs leave.
As I mentioned in my post, the photo processing tools on my belt. One program I use is the GIMP, a free image processing program, with pretty much the same capabilities as Photoshop.
RAW images, which are just output from sensor data, are often a bit soft. And they need to have some sharpness applied to the edges to create a crisp, realistic look. Some software has a “clarity” function that does this nicely. But the “sharpen” function in most software, a method called, ironically, “unsharp mask”. You can adjust to make the effect occur only at the edges, but, even so, they often wind up with jaggy artifacts.
High pass sharpening is a technique that can be used in both Photoshop and the GIMP. Here is an article explaining how the method works in Photoshop and a little about the theory. My check list directions below allow you to easily use this same technique in the GIMP, a free photo editing program.
High-pass filter sharpening method for the GIMP
- Load the file into GIMP.
- Filters>Enhance>Denoise, if you haven’t already. This process can emphasize noise as well as edges, so reducing noise before you start can make results much nicer.
- Make a duplicate layer (Layers>duplicate).
- Create a high pass filter:Colors>Desaturate. Experiment with the options for method (Luminance, Luma, Lightness, HSI, HSV) and select the one that give the clearest detail where you want the detail improved.
- Filters>Enhance>High Pass
- Change the blend mode of the High Pass Filter layer to one within the Overlay set (overlay, soft light, hard light, vibrant light, pin light, linear light…) Experiment to find the one that gives you the best effect.
- Adjust to your taste: If you want more clarity you can duplicate the high pass filter layer, for less you can reduce the opacity of the layer.
Before 
After
Notes:
You can insert adjusting for general exposure between steps 1 and 2.
You can use layer masks to apply the high pass filter to only a portion of the image.