All photographers know about vignetting.  That's where you darken the outer regions of the image, to help draw attention to the center of the image.  The fastest way to do it is to make a selection around the area of interest, invert the selection so the outer region is selected, feather the selection if it isn't already, make sure the background color is black, and press the delete key.

But that method, while fast, has some drawbacks, mainly that it can't be adjusted.  It's all or nothing.  What you can do, instead of simply pressing the delete key, is open the Levels dialog, slide the right-hand Output Levels slider all the way over to the left, and press OK.  Now the Edit Fade command is available, and you can adjust the results to your liking.

Well, there is still a drawback -- the Edit Fade command, while extremely useful, can't be used again if you enter another command.  What if you did several other things, and then wanted to come back and re-adjust the vignetting result?  One way of handling that dilemma is to duplicate the layer before you go through the vignetting process.  You still make the selection, invert, feather, and either press the delete key or use the Levels command -- but you do it on the lower level.  The upper level contains all the information available before you began.  You can make changes any changes you want to the upper level, and then reduce its opacity to the desired amount.

Well, that's not so good, either.  When you reduce the opacity you are combining two images, one of which has been changed in some way, the other not.  We should find another way.  A recent email from David Ziser highlights a method he uses for vignetting, which we can build on to make a really useful tool.  It uses two layers, but all the photographic information is on one layer, and all the vignetting is all on the other layer.  Let's see how it works.

You start by creating a new, empty layer.  With that layer selected, click on the Rectangular Marquee Tool and adjust its Feather setting to 250 px.  Make a rectangular selection leaving about a half-inch border, and then press Ctrl-Shift-I to invert the selection.

Make sure the Foreground Color is set to black (press Ctrl-D for "default").  Now click the Paint Bucket Tool, and then click twice in the selected area on the image.

You should get something like that looks like this:
 

Note that the layer with dark shading (Layer 1) lies above the original image.  You can adjust the Opacity of Layer 1 to get just the vignetting strength you want.  You can alter the Background layer any way you like, without disturbing the vignetting.  Also, you could have used the Elliptical Marquee Tool (which is Ziser's choice) and come in closer around the primary area of interest.

But wait!  There's more!

Look carefully at the Layers Palette shown above; you'll see that the Blending Mode of Layer 1 is set to Luminosity.  This is a precaution, because sometimes the blending of black with the image colors can produce an undesirable color cast.  Using Luminosity for the Blending Mode will prevent this (I'm indebted to Dale Kincaid for suggesting this cool trick!).

But what if you wanted a really pretty color, but you had already created the vignette layer using black?  Not to worry, The Photoshop Corner will help you.  This is where we take Ziser's method to the next level (no pun intended).

With Layer 1 selected, create a New Adjustment Layer and make it a Selective Color layer.  Click OK on the dialog that results, and then press Alt + Ctrl + G to create a Clipping Mask for the Selective Color layer.  This makes any changes from the Selective Color layer apply only to the layer right below, namely, the vignette layer (Layer 1).

Now double-click on the Layer Thumbnail for the Selective Color layer.  A dialog will open up.  Go to the Blacks channel, and fiddle with the Cyan, Magenta, and Yellow scrollbars a bit till you get a combination you like, and then click OK.

Notice the pleasant vignette you get, without disturbing the colors of the original image!  In this case we let the opacity of the vignette layer stay at 100%.  (Actually, the fourth [Black] slider effectively alters the opacity for us.)  If you particularly like the result, you can save the combination by pressing the "Save..." button.  It will get saved as an ASV file, and you can "Load..." it later on for another vignette.

There's lots more we could do, but maybe another day...