By David Rosenthal.
Sharpening is an important tool in your darkroom kit. It can be daunting and seem complicated. But it doesn't need to be. There is one thing to remember about sharpening above all else: it's to taste. Your taste. In this tutorial we're going to apply sharpening to a duplicate layer. To make a duplicate layer, type Command-j (Mac) or Control-j (PC).
When you make a new layer it shows up in your Layers Palette, called Layer 1. You can rename it, if you like, but what we're really interested in is the little eyeball button to the left of it, which the arrow's pointing at.
This eyeball turns the layer on and off. If you can see the eyeball, then you can see the layer. If you can't see the eyeball, the layer is turned off, or is invisible.
More on this later.
For now make sure that layer is selected and choose Filter>Sharpen>Unsharp Mask.
The first image (left) that we're going to work with has been resized to 800x533 pixels. For that size image the settings that I've put in (Amount: 150, Radius: .4, and Threshold: 0) work pretty well.
A couple of things to note. You can turn the preview button on/off to see the effect on the opened file, or you can click in the smaller window in the filter dialog box to turn off the sharpening temporarily. It's good to see the difference with and without sharpening...
Once you get it set where you want it, hit OK
Now that you've added sharpening to the top layer, you can still make changes to how much sharpening there is. At this point, you can only reduce the amount.
1) You can toggle the eyeball on/off to see the effect of the sharpening.
2) You can play with the opacity slider (that's the new arrow that just showed up) to control how transparent this layer is. The more transparent the layer, the less you see of it, the less effect the sharpening has on the image.
This is an animated view of a 100% crop of this image. Watch as it toggles back and forth between the unsharpened version and the sharpened version.
As you can see, the larger the image the larger the Amount of sharpening needs to be.
Play around with some of your own images. Watch as you oversharpen and you notice white and black halos that get added from too much sharpening (again, to taste!). Play with the sliders and see what they do. But mostly , don't worry too much. Have fun. As long as you're working on a duplicate layer you can always go back and scale back on the amount of sharpening, or delete that layer all together and start all over.