How Postproduction Sharpening Filters Work
Sharpening filters work in postproduction by emphasizing the edges (or adjacent areas with tonal contrast to one another) in an image. They create a fine highlight on the darker side of the edge, and a tiny lowlight on the lighter side of the edge.
This added contrast causes the edges to appear crisper from a distance, but when viewed up close, you can see a halo of sorts. This photograph provides a magnified example of what an image looks like after being sharpened in postproduction.
100mm, 1/160, f/8, 400
A reliable sharpening tool (such as the UnSharp Mask Filter) provides three variable settings for you to adjust depending on the size of your image and how sharp you wish it to be. These three settings are:
Amount controls the level of contrast created by the sharpening tool. A higher amount of contrast creates a greater difference between the highlight and lowlight edges.
The radius determines the width of the highlights and lowlights created by the sharpening filter. You should choose the radius for a particular image based on what you see in the tool’s preview window (by clicking on the preview window, you can toggle the effect on and off to see just what it’s doing to the image).
An image with a higher resolution requires a greater radius than one with a lower resolution. Higher radius values can cause visible halos at the edges, so it’s important not to get too carried away with this one, or it’ll be obvious that you applied postproduction sharpening to your image.
Threshold limits how much of the image is affected by the sharpening filter. It defines what is and isn’t an edge by setting a standard of how far apart two adjacent areas must be in tonal value before they’re affected by the radius and amount.
In regards to threshold, high-contrast edges are more likely to be affected than smoother areas. A low value causes more of the image to be affected, and a high value causes less to be affected (protecting smoother surfaces in the image). One reason to choose a higher value would be to protect a person’s skin from being sharpened, which can cause the skin to look rough and overly textured.