nik Sharpener Pro!
As discussed in my PhotoShop tutorial Instant PhotoShop, there is a problem with Sharpening (and not just one). The one that I'm referring to at the moment is that the Unsharp Mask filter can not be done on an Adjustment Layer. This means that once the file is saved after Sharpening there is no way to undo it. (If you don't understand or appreciate the usefulness of Adjustment Layers, please read the above tutorial.)
Consequently many PhotoShop users do all of their image processing adjustments without using Unsharp Masking and instead save the file unsharpened. Then, when they are ready to print they flatten the image, (merge the Adjustment Layers) apply the Unsharp Mask, and then print.
This is the preferred way of working, but it doesn't address all of the other problems associated with the use of Unsharp Mask, the preferred sharpening tool in PhotoShop.
Sharpening is one of the most danger fraught activities in image processing. Too much or too little and the image can be ruined. But, what are the proper settings for Threshold, Percentage and Radius? I have no idea, and unless you're willing and able to do some complex math, neither do you. What most photographers do is to use a setting that has worked well for them in the past, or they struggle ‹ looking at the screen trying to decide which slider to slide, and by how much.
Looks OK? Maybe. But what you're doing is sharpening the image for a 72 dpi screen, not for the size and resolution of the final print. Making an 8X10" today and a 13X19" tomorrow? Sorry! Once your file is sharpened and saved that way, that's it. You have to go back to the original scan and redo everything if you want change it. Want to upload a particular shot to the net? What about preparing it for commercial printing? Who knows what the right settings are? This is not a good thing, to paraphrase Martha.
An Automated Solution
nik Sharpener Pro! is a superb product that solves this problem. Let's see what it does, and why.
nik Sharpener Pro! (NSP from now on) is a PhotoShop plugin. (This means that it'll work with any image processing program that accepts the Adobe plugin standard.) The workflow is to scan and process your image as you normally would but ensuring that no sharpening of any kind is applied by your scanner or any other program. When done, save the file including any Adjustment Layers.
When ready to print load the file and set the print size and resolution that you plan on using for this particular print. If you've used Adjustment Layers, make sure that the Background Layer is selected.
When you're ready to print, Instead of running PhotoShop's Unsharp Mask, run the NSP filter from under the Filter menu in PhotoShop. You'll discover that there are separate filters for Inkjet, Color Laser, Internet and Offset printers.
The program will ask you a number of questions about the image, including its size, and the type of printer resolution available. You'll even be able to specify at what distance the viewer will likely be viewing the print. (An 11X14" print hanging in a frame on a wall will need different sharpening than one in the hand at close inspection.)
A Preview screen is available, but as the developer points out in the manual what you see onscreen is not what you're going to get on paper.
According to the manual there are quite a number of special features that allow NSP to do a much better job than Unsharp Mask ever could, even if you knew exactly what settings were needed for a particular print size. Among these are the ability to apply different amounts of sharpening to different areas of the image. So, for example, a sky area probably doesn't need as much sharpening as a field of grass, and NSP therefore deferentially applies varying amounts to each area.
The program also features Fence 'n Foliage protection. This prevents over-sharpening on areas of the image that have smaller high-contrast transitions than others.
Finally Hue Protection prevents the colour and contrast shifts that occur when an Unsharp Mask is applied to a colour image in other than Lab / Luminance mode.
Unfortunately this program is very expensive at $329 US. (There is a less expensive version, but nowhere near as full-featured.) The way I rationalized the purchase was that since I will likely use it on thousands of images over quite a few years, the opportunity cost turns out to be quite reasonable. I hope that you come to the same conclusion. This is a very useful tool indeed.