1. Selective sharpening using layer masks

The unsharp mask tutorial explained how to use the unsharp mask filter, but it has it's limitations as the whole image is sharpened the same amount - globally. However, wouldn't it be good if we could selectively sharpen just the parts of the image that we wanted to? For example, we could sharpen the subject, but not the background to make it stand out a little more. Also if the background is a smooth sky, we don't want to sharpen the grain in it too. With the selective sharpening technique I am about to describe, you can literally paint-on the sharpening just where you want it and to the desired amount !

Here is an image of a robin - a nice sharp shot, but I would like to selectively sharpen it's feet and the post it is standing on a little without sharpening the bird or the background any more. The first step is to open up the layers palette (keyboard shortcut F7) and to make a copy of the background layer (control + J).

Next we are going to globally sharpen this duplicate layer using the unsharp mask tool as usual.
It is best to view at 100% magnification (actual pixels) to gauge the effect, and then click filter > sharpen > unsharp mask. Use the sliders to adjust the degree of sharpening as described in detail in the unsharp mask tutorial.
It is ok to over sharpen a little as we will moderate the opacity (and hence the amount of sharpening) of this layer later.

Now looking once again at the image above - look at the bottom of the layers palette. There is an icon (second in from the left) which is a little grey rectangle with a circle in it - this is the add layer mask icon. Hold down the Alt key and simultaneously click on the add layer mask icon. Look again at the new layer box and you will see that a black box (layer mask) has appeared over this layer. Also the sharpening that was previously applied has now disappeared again.

So what is going on ? well, we created a new identical layer over the existing background layer and then applied sharpening to this using the USM filter. When we held down the Alt key while clicking on the add layer mask icon, so a black mask was placed over the second (sharpened) layer - making it invisible - which is why the sharpening effect disappeared. All that needs to be done now is to select a soft brush of suitable size, and with white as the foreground colour, paint the sharpening back in just where we want it.

So, looking at the image above, notice that I have selected a paintbrush from the tools, and in the top toolbar you will see that I have selected an opacity of 30% and a flow of 30%. I will therefore not be applying all the sharpening at once, but I can gradually build it up to the desired level. Notice also that I have set the foreground colour to white before painting on the sharpening. (This is done by ensuring that the two rectangles in the tools to the left of the screen are arranged white over black. If they are the other way round, click on the little curved, two-headed arrow to reverse them). The effect will not work if you have black selected as the foreground colour.

Next, working with a suitably sized brush (use the square bracket keyboard keys to alter) paint on the image where you want the sharpening to be applied. In this example I have just painted over the bird's feet and the post. To paint, hold down the left mouse button and move the circle over the area to be sharpened. You will notice that a white hole in the black layer mask is appearing in the layer mask dialogue box where you are painting over the black mask with white - making it transparent again. If you want to move the area around that you are working on, hold down the spacebar and a hand will appear on the image and you can move it around with the mouse (whilst simultaneously holding the left mouse click button down)

When you have finished, click on the zoom tool and select "fit on screen" to see the result of your handiwork. Finally, select layer > flatten image and then save your image as usual.

This is a very powerful technique - enjoy.

2. PROGRESSIVE SHARPENING

It is possible to get smoother sharpening if it is done progressively in small steps rather than in one big step. e.g. To obtain 300% sharpening, sharpen three times using an amount of 100% without changing the the radius and threshold sliders.

3. LUMINENCE SHARPENING

This is a useful technique to try on difficult images where artifacts are proving hard to avoid.
Some tutors advise converting the image to lab mode, selecting the luminance channel only, apply the USM filter and then convert back to RGB again. This is unnecessarily long-winded though. Try this method instead:

Apply the Unsharp mask filter as usual, but intentionally over-sharpen a little. Next select the Fade filter from the Edit menu and change the blending mode from Normal to Luminance. Then using the slider, reduce the sharpening with the slider to the correct amount. This has the same effect of changing to and from lab mode.

4. SHARPENING SELECTED CHANNELS

The blue channel often tends to contain most digital noise, and so it can be worth trying to sharpen the green and red channels only.

To do this, select window > channels, click on the red channel and then shift +click on the green channel to select this too. The image should look very yellow as blue is missing. Now click in the small empty square to the left of the RGB layer, an eyeball icon will appear and you will be able to view in RGB colour again, but notice that the red and green channels only are selected. When the USM box appears, the image is still yellow, (as in the image below) but you can still gauge the effect on the RGB colour image in the main window.

When you are happy, click ok and then select RGB again in the channels palette to return the image to normal.

5. A FINAL ALTERNATIVE

There are numerous third-party sharpening plug-ins that can be purchased which apply a sophisticated series of actions to sharpen images without aggravating noise and with minimal halos.One such tool that I often use, particularly on high- iso or other "problem" images is called Intellisharpen from Fred Miranda. This has high and low ISO sharpening algorithms with or without noise reduction and levels tweaks. There are versions tailored for various makes and models of camera and I find that they are not only very effective, but are easy to use and good value too.

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