Edge Sharpening Thoughts

Fresno Bonsai Exhibition, Nikon D810 with Sigma 35mm f/1.4 Art Lens at f/4.8 1/180 sec ISO 64 handheld

by Rich Seiling

Edge sharpening has become a popular technique, but I think it’s better for film scans than for digital camera images. Let me tell you why, and how it can be adapted for digital camera files. 

Edge sharpening goes way back. I first learned to do it around 1999 from Bill Atkinson, one of the first Apple employees and the reason windows in the mac interface work the way the way they do , and the mouse, and a whole host of other patents. 

Atkinson created an edge sharpening action script for Photoshop because of the grain in film scans. When you sharpen a film scan, you also sharpen the grain. Edges are more tolerant of heavy sharpening than flat even toned areas like sky. So Atkinson introduced people like Galen Rowell and a host of other well known landscape photographer to this technique, and it made it’s way into our toolbox. Atkinson’s tool has been an incredibly important contribution to the art of printmaking.

But fast forward to today. Is sharpen edges still the right technique? Low ISO digital images do not suffer from the grain problem of film. And edges are already sharp because of the nature of digital capture.

I still use a variation of Atkinson’s script today, but I use it inside out. I find that sharp edges in a good digital capture need little, if any, additional sharpening. It’s easy to make them halo and crunchy, and I don’t like that look. But smooth tonal areas often suffer from a lack of apparent sharpness. Part of this is because of the Bayer filter in our digital cameras. The pixels in our photos are only seen through one color filter, and the actual color is reconstructed by interpolating neighboring pixels viewed through different color filters. This does some stuff to non edge/smoother tone areas that degrades the apparent sharpness.  

Here’s an edge mask I created from the red channel of the bonsai photo.

Edge Mask

My approach with digital camera images is to use the sharpen only edges scrip to select the edges, and then I invert that selection so that I can sharpen only non-edge areas, which I find can take much more sharpening than the edges. 

These gifs show a 100% actual pixels section of these images with no sharpening, edge sharpening, and non-edge sharpening using the settings below:

In the top photo, look at how the edges of the leaves and the texture of the tapestry on the right respond to different sharpening approaches. And in the bottom photo, again look at the leaves, as well as the woven grass mat and the edges and detail in the wood stand.

Sharpening settings used for edges and non-edges.

As you can see, the non-edge areas handle this amount of sharpening better than the edges. In fact, with edge sharpening, I find the edges of the leaves over-sharpened at this setting, and part of the wood stand is over-sharpened as well.

Do I sharpen every image this way? No. But I do use this technique a lot, and the understanding of how and why is what guides my choice of tools. How, and how much to sharpen is very much a preference. Everyone has a different flavor. Lately I find myself applying less sharpening than in the past, and trying to closely replicate the kind of image quality and appearance you see in large format prints by photographers like Ansel Adams. How you use the tool is up to you!