Adobe Photoshop Lightroom has become my preferred tool for making digital vignettes, but almost every editing program has its own special tool designed for this very purpose. Lightroom's most powerful vignette tool is called Post-Crop Vignetting. Sounds perfect, right? It's a set of sliders found under the Effects heading in Lightroom 4's Develop module. It's pretty straightforward, too. Simply slide the Amount slider higher for a light vignette (not something I often recommend) or to a lower value for a dark vignette. The real brilliance of the tool, however, comes from all of the control it provides for fine-tuning the look of the vignette.
The midpoint slider, for instance, determines whether the vignette begins closer to the center of the frame or close to the edges. The Roundness slider adjusts the vignette's shape—from round to oval and even almost rectangular in the extreme. Feather is perhaps the most important slider to use with Post-Crop Vignetting, as it softens the transition from the original photo to the darkened edge. Too much feather can make the vignette all but invisible, while too little makes for an unpleasing, hard-edged transition. This is perhaps the biggest key to creating a pro-caliber vignette: subtlety. (There's no right way for every vignette, but I like a fairly strong feather to keep the edge smooth and help hide the transition.) Lastly, is the Highlights slider that adjusts how much of an effect the vignette will have on highlights in the image. It can be very useful to make the vignette appear to fall behind a light-toned subject—creating the same effect as a lighting-style vignette without appearing overly obvious as post-production
There's another great way to create a vignette in Lightroom, and that's the use of graduated filters. While not a true "vignette" (because it's not uniformly round) the effect it produces is similar because it's used to darken the edges of the frame and push a viewer's eyes to the center of interest. I like to use a graduated filter with the exposure set at 1/3-, 1/2-, or even a full stop underexposed, then I position it at the bottom of the frame to draw the eye away from this relatively low-interest part of the picture and up toward where the interest is. I frequently use a few graduated filters in conjunction, from different positions in the frame, to recreate the same basic symmetrical shape as a true vignette, but with more local control as to its precise placement within the frame.
For those who prefer using Photoshop to create digital vignettes, there are plenty of opportunities for customized shapes and patterns in your vignettes. The classic oval vignette, though, can be best accomplished by combining adjustment layers with a feathered oval mask. To start, choose an adjustment layer that affects the overall brightness of the scene. This could be Curves, Levels or Brightness/Contrast, but I like to start with Exposure. Apply a fairly strong underexposure to the scene—maybe 1.5 or even 2 stops underexposure. It should look dark, but not yet a vignette.
With the image now appearing well underexposed, click on the mask thumbnail in the Layers menu to activate the layer mask. Then select the oval marquee tool, and click in the top left corner of the scene and hold the mouse click. Next, drag the marquee all the way to the bottom right before releasing it. You'll now be faced with oval-shaped marching ants representing the selected area of the mask. Next, to feather that selection, choose Select>Modify>Feather and set it to the maximum 250-pixel option. Then choose the paint bucket Fill tool, make sure the foreground color is set to black, and click in the center of the selection to fill the mask and reveal the original image layer.
Experimenting with different feather amounts, layer opacities and selection areas can provide complete control over exactly where you paint your vignette, and how well it conforms to the particular shapes in the image. When in doubt, though, a subtle, soft oval vignette is a great place to start. If you're unsure of the effect, try clicking the layer on and off for a before and after view. It should instantly provide proof that this simple tool is also very powerful.