It is a fact of life that nearly all digital images need a bit of sharpening and Photoshop (PS) gives us several great tools for doing that. We even find some sharpening tools in Adobe Camera Raw (ACR) but these are not the same as the tools we find in PS, nor do they serve the same purpose. As we make our adjustments we make actually create softness and noise that did not exist in the original image. This tools in ACR were made to deal with these artifacts before sending them over to PS.
Sharpening
Imagine for a minute a dark tree trunk against bright snow, the tree is black the snow white. For the most part our image sensors have no trouble faithfully reproducing this scene, however, what happens to the pixels on the sensor that are to show the edges of the scene where tree is against snow; half of the pixel is white, half is black? What value does the pixel report to the processor? You guessed it, gray. So instead of this sharp contrast of black against white as in the original scene, we have a soft gray line between the two. Can we get rid of this? No, but we can play a trick on our eyes by making it look like we got rid of it by increasing the contrast on either side of the line. Make the white a bit whiter and the black a bit darker and the overall effect looks like we have sharpened the line. This is the priniciple that image sharpening works on. Furthermore, as has been mentioned, the effect of working the image in ACR can cause its own softening, requiring a re-sharpening before sending to PS.
I've cranked up the Amount to where it looks good but it should be obvious to everyone that I am having a problem with Noise.
Under the sharpening tab, the one with two triangles, you will see four sliders under Sharpening and two more under Noise. Now before I tell you how to operate these sliders, increase your image to at least 100%. Go ahead, do it now because this won't work unless you are at least 100% magnification. Now, press down the ALT key and grab the first slider marked Amount. (Option key for you Mac users.) You will see your image turn black and white. This is to allow you to better see the effects of sharpening as you work the slider. Since this is Amount, we are controlling how much sharpening we are applying. When you get it where you like it, we'll go to the next one.
The next slider is marked Radius and it determines the ramp up to the contrasty area on the edge. If you want razor sharpness, leave it at zero, but that may not be good for portraits. A radius of 3 pixels will soften the effect a bit without sacrificing the sharpening you just did. Again, pressing and holding the ALT key allows you to preview your work.
The next two sliders, Details and Masking control what all gets sharpened. Work the Details slider with the ALT key pressed and you will see all the nooks and crannys that get emphasized as you push the slider to the left: good for insects, not so good for grandma.
I have the details down to one and the masking all the way up, but I am still having a noise problem.
Masking is amazing. REmember that the rule of thumb in PS masking is that white reveals, black conceals. So when you press the Masking slider, with the ALT button pressed, the whole scene is white because sharpening is being applied everywhere evenly. As you move the slider left you will find that black areas appear, masking off certain areas from sharpening until, all the way to the right, you see just an outline of your image with sharpening being applied only to the edges. How do you like that? I told you it was an amazing tool!
Noise
Again, in the process of working with this image some noise may have been generated. Noise is the digital equivalent of static on your radio. It usually appears in the shadows and sometimes in the brights; rarely in the midtones. The program that deals with noise tends to also soften the image, which means it is working in opposition to what you just completed in sharpening. Unfortunately, you often have to find a happy compromise between the two.
Here I have reached a compromise. I couldn't get rid of all the noise so I had to back of on the sharpening to reduce noise levels to a more acceptable level. I'll have to finish up in PS. You won't usually have this much trouble.
Most of the noise is going to appear in the Luminance channel. (This is equivalent to the "a" channel if you have ever worked in LAB color.) This is where you are going to do most of the noise reduction. Center on a dark area, move the slider until is just gets rid of the noise but no more, you want as little interference with sharpening as possible. If you find that most of the noise goes away but there is still some residual noise, try the color noise sldier. Again this is accomplished by softening the color channels and should be used as little as possible.
Keep in mind, both sharpening and noise in ACR are for handling internal problems. So, what do you do if you are not happy with the results of these two systems? Go ahead and move the image over to PS where you have much more powerful tools for dealing with both problems.
Final image after further noise reduction and sharpening in PS. Not bad. Of course your looking at it on a computer screen, it looks even better in print.