… read what Bruce Fraser, one of the program’s designers has to say about Output Sharpening.

In the capture and creative sharpening phases, it’s usually desirable to avoid obvious sharpening haloes. With output sharpening, though, that’s not the case. The rule of thumb I use (one that has held up well under a good deal of empirical testing) is to aim for a sharpening halo that’s somewhere between 1/50th and 1/100th of an inch in width—the thinking being that at normal viewing distances it’s too small to see as an actual halo.To accomplish this, you need to know the output size and resolution, and the way in which the output device converts the pixels into marks on paper. For example, if you’re printing to an inkjet printer at 300 ppi, you want to create a sharpening halo that’s about 3 pixels wide. If you’re printing to a halftone printer such as a press, using a 266-ppi file and a 133-line screen, you need a halo that’s at least 4 pixels wide, because each halftone dot is made up of four pixels, and you may well be rewarded if you make the halo 5 or 6 pixels wide. Needless to say (but I’ll say it anyway), the result will look pretty ugly on screen when viewed at an Actual Pixels zoom level. For years I’ve advised people to make the file look “crunchy” on screen when sharpening for offset: my research suggests that I was understating the case. You can make better judgments about output sharpening by viewing at 50% or even 25% (avoid the “odd” percentages like 66.6% because Photoshop applies heavy antialiasing to those views), but the only truly reliable way to make the judgment is to view the printed result. If you follow the above-mentioned formula, keeping the haloes to between 1/50th and 1/100th of an inch on the final output, the results will look good.

Now you know why I wanted you to read what Bruce had to say before showing you this image. Figure 12 shows a center section at 100% with Output Sharpening. Over the top, right? But not on a print! On a print made with an Epson 2200 the output just sings!

- Michael Reichmann
PhotoKit Sharpener 1.0: A Complete Sharpening Workflow