It can be very subtle, it can be very obvious. One thing is for sure: Vignettes make pictures better... most of the time.
Back in the days of film, vignettes were actually an unwanted and undesirable effect caused by lenses and the film. However it's removal sometimes proved to be... bad... so you may want to know how to apply one.
WHAT'S A VIGNETTE?
Pronounced "vinnyet" (not vignet), a vignette is basically those darkened corners you see in images. If you're attentive, you'll notice that almost all of my images (and ALL of my manipulations) have vignettes.
WHY WOULD I WANT THAT?
Well, like I said it's a really old-school thing, which immediately means that it's crucial for that vintage look. Secondly, it helps keep attention in the middle of the image. Say if it's a potrait or a landscape with the subject in the center, vignettes force the viewer to keep their eyes in the center of the image.
In design (like for wallpapers and stuff), it also keeps the eyes in the center and it also looks aesthetically pleasing to the eye. It's also a great way to hide errors in the corners/edges of the image.
APPLYING VIGNETTES IN PHOTOSHOP
There's actually quite a few ways to do it:
1. Go to Filter > Distort > Lens Correction...
There's a vignette setting at the very bottom.
2. Get out the Elliptical Marquee (M) and draw out a selection from the top-left to bottom-right corner of the page. We basically want the image to have an ellipse inside with it's edges touching the edges of the page. Then you can do one of the following:
a. Go to Window > Actions then expand "Default Actions", click on "Vignette (selection)" and press the Play button at the bottom.
b. Create a new Solid Color and make the color BLACK. Press Ctrl+I to invert the layer mask created then go to Filter > Blur > Box Blur... and apply the amount required (it depends on the document dimensions). You may have to tweak the opacity of the layer.
c. Create your own action to merge a. and b. :-)
I may or may not explain how to do this process during my tutorials, but it's good to have it here just in case.