I find that almost every one of my images benefit from some amount of "artificial" vignetting—a darkening of the periphery of the image. I must qualify that observation by noting that, like every technique, if it is overdone, or performed indiscriminately regardless of the image, then it becomes a gimmick. Worse yet is when it isn't even done properly. There are a number of aspects to take into consideration.
A quick and easy way to go about it
First remove the natural and optical vignetting present at capture. This is important because that "genuine" vignetting varies according to the camera/lens/aperture combination, which is no guarantee that it will correspond to the vignetting you'll ultimately want for the image—I find it easier to work on an image that has first been "normalized", and removing the initial "surprise" vignetting is part of that process. Perhaps even more important is that the original vignetting is tied to the frame captured—but if you crop your image, then the vignetting won't be linked properly to the final composition.
Notice how the vignetting of the original frame no longer makes sense for the cropped frame.
If you are using a conventional lens (e.g.: not a TS), Lightroom 3 (or another raw image processor that supports a similar feature) can normalize the image easily enough by using a lens correction profile—which will also take care of barrel or pincushion distortion and a certain kind of chromatic aberration for you (an invaluable time-saver).
After you have cropped the image, ideally towards the last steps in the image processing, introduce a "post-crop vignetting" to the image (again using Lightroom as an example, but it is by no means the only software that offers this).
Points for consideration
- In Lightroom version 1, there was no way to apply the vignetting to the cropped composition—in fact, the vignetting control was squarely aimed at correcting the naturally-occurring one, not the creatively introduced one. Luckily, a separate, "post-crop" vignetting control was introduced with Lightroom 2.
- The way the "darkening" is applied was greatly enhanced in Lightroom 3, with its "Highlight Priority" feature. It used to be that the vignetting was basically a black gradient applied over the image with a certain opacity, which had a disastrous result in the highlight areas. Indeed, vignetting doesn't look natural if it indiscriminately darkens the whites and turns them gray.
No vignetting (left), "Highlight Priority" vignetting (center), "Paint Overlay" vignetting (right).
Notice how, when the highlights are not protected, the whites become dirty gray and unnatural looking.
(Amount exaggerated for illustration purposes, but it is the same amount in both latter cases.)
- It only makes sense to apply the post-crop vignetting at the raw processing step if you do not intend to perform additional work on the image in Photoshop. If you "bake" the vignetting onto the image and then go about working further on the image, not only won't you be able to easily change the nature and amount of the vignetting (have fun!), you'll have to work around the uneven illumination it introduced—and it is harder to clone pixels around if the source and destination are not of the same brightness.
- Finally, it is nice that Lightroom allows certain controls over the nature of the vignetting (midpoint, roundness and feather), but who says the image you're working on requires an indiscriminately, evenly distributed "darkening" around the middle of the frame? Maybe you'd like to "protect" some regions from darkening, or would like the emphasis to be around a point that is not smack in the middle of the frame? Unfortunately, in this case you cannot use the easy solution provided by the raw processor.
For those various reasons, unless I'm quickly throwing together a preview, working on snapshots, or am lucky enough to be working on an image that doesn't need any additional work and special vignetting considerations, I won't apply the vignetting in Lightroom at all and will do that later in Photoshop.
Gaining Complete Control
One way to go about vignetting is to manually "burn" the edges with a brush (on a separate, adjustment layer, so as to not "bake" the transformation in the source image). A good approach here is to place an adjustment layer on top of the image (any effect—doesn't matter, since you won't actually be using its features), put it in the Multiply blending mode with a black layer mask, and simply paint white with a large soft brush in the mask to "burn" at will.
(Any) adjustment layer placed in Multiply blending mode, using white in its mask to burn the image at will.
This approach is fine and allows complete control, but I find it hard to keep a steady hand and make a clean, soft and even gradient.
The approach I developed is slightly convoluted, but it allows a good mix of parameterization (which saves a lot of time) while at the same time allowing customization.
- With the swatches set to black/white (press 'd' to reset), create a Gradient Fill Layer on top of the image. The Gradient Fill dialog box opens.
- Set the style to "Radial" (obviously), check the "Reverse" box, and enter a 150% scale. Already, you begin to see where this is going...
The Gradient Fill's starting point.
Now is a good time to point out that, while the dialog box is open, if you click on the image, you can drag around the center point of the gradient. This allows you to put the focal point of the image anywhere, not necessarily in the middle of the frame. This is very cool and is much, much simpler to tweak than having to "paint" pixels in a mask to do the same thing. Remember that, since this is a parametric layer, you can come back later and move that around at will without destructing anything.
- By default, the gradient is linear and goes from 100% transparent (in the middle) to 100% opaque (on the edge), starting its ramp immediately. This is typically not what you'll want, as you'll usually want to darken only on the edges of the image, not all the way to the center of the frame. To fix this, we need to go edit the gradient, by clicking on it. In the dialog box that opens, I typically start by placing the 0% opacity stop at around the 75% location instead of the end. This means that a portion in the middle of the frame won't have any darkening applied.
The Gradient Editor allows control over how far the darkening will occur towards the middle of the frame.
- Press OK to close both dialog boxes. The next step is to change the blending mode of our new gradient fill layer to Soft Light (this will provide the "highlight priority" effect needed to not produce ugly, dirty gray whites) and adjust the opacity to change how much vignetting you want (I typically start at around 30%).
- You now have a great starting point of even, soft darkening goodness, but this is just the basic setup. The layer still has a blank mask available for further customization. I use this mask to avoid darkening regions of the image that do not need it or shouldn't have it.
- In a landscape, I might want to vignette the bright blue sky much more than the already darker ground, so I'll mask out most of the bottom part of the gradient layer.
- In an architectural shot, I will want to avoid darkening the extremities of a building that reach close to the edge of the frame, so that the sky has a nice gradation, but I am not actually darkening the star of the shot.
- In a portrait, I will make sure the face doesn't get darkened by the vignette, further focusing the attention how I'd like it.
The great thing about this approach is that even with the mask painted properly (to protect regions that shouldn't be affected as much by the vignette), you can still go back and adjust the opacity (which is the equivalent of the Amount, in Lightroom), edit the gradient's opacity stops (to change the equivalent of the Midpoint and Feather), and even move around the center point of the gradient if the focus is not in the center of the frame.
Needless to say, if you find the creation of that gradient layer tedious, you can encapsulate the whole process in a simple Action, so that it takes a second to add a powerful vignette to any image, that you can then further customize.