A good look at sharpening using Photoshop's High Pass Filter

Working with a photo of a fly, which I took just for fun and ended up using in a poster, I discovered how to use the High Pass filter to sharpen. I followed along in the chapter of Photoshop CC: The Missing Manual, called The Art of Sharpening.

Many techniques occur in this chapter, including noise reduction and several ways to sharpen edges. Given I chose this photo which has an overall lack of sharp edges, only one method worked.

The image is below, taken handheld with a manual focus macro lens. The subject was simply a fly on my kitchen counter, where he stayed for quite a while. I was experimenting with different angles and wishing for an unusual shot for posters or collages.

Click on an image to see a larger version of all the photos in this post.

Original fly before sharpening

Fly after sharpening with High Pass Filter

I had heard many times that the High Pass filter is great for sharpening. This book explains it better than any other sources I've seen. Midtones become more contrasty, unlike the Unsharp Mask method that everybody loves. Unsharp Mask makes light pixels at an edge lighter, and dark pixels darker.

The High Pass filter instead finds biggest areas of difference between colours (essentially in the midtones) and increases their contrast. This layer is run on a duplicate layer; it becomes grey except for newly emphasized edges, so you then choose the Overlay blend mode. 

The result is quite amazing. Look closely and you'll see in the photos above, that the sharpened fly has crisper edges throughout its face and body. The difference is subtle but no other sharpening method came close to sharpening the odd features on this fly's head.

Another time I'd like to try using the various sharpening methods on a daytime around-the-town photo, more typical of the average internet user's snapshots, and see what the differences are. The reason I've not blogged much with my Photoshop techniques is that my photos have not been imported into my new computer. Now I have a great reason to do that! But for now, I'm impressed.

UPDATE:
After I posted this, I repeated the High Pass filter on a contrasty household snapshot and got a better sense of how this filter works. It's a more typical sample of results you may obtain with your own photos of easier-to-photograph subjects.

Dry erase brush before high pass filter

After high pass filter with mild settings added

After high pass filter with drastic settings added; notice halo at edges

This example shows how the High Pass filter affects ordinary objects including print. In the original, dust wasn't evident on the ledge, but as the filter was added, it becomes noticeable, and the text gets sharper. At some point, a halo is introduced where sharpening becomes counter-productive except for funky effects perhaps.