People often use the vignetting sliders in the Lens Corrections panel (or Lens Correction Vignetting in the Vignettes panel of Lightroom 2)  to darken or lighten the edges of their photos for creative purposes.

Is there anything wrong with this?  If it looks good, then definitely not.

However, this Lens Corrections vignetting is designed specifically for eliminating dark corner vignetting that many camera lenses create.  It works on the edges of your original photo.

Post-crop vignetting, on the other hand, found in the Effects panel of Lightroom 3 (and under Post-Crop Vignetting in the Vignettes panel in Lightroom 2), works on the edges of the photo, with any cropping you may have done.

Below is an example of applying each type of vignetting to darken the corners.  I have cropped the right hand side of this photo out, to make a square:

Lens Correction Vignetting

Post Crop Vignetting in Lightroom’s Effects Panel

So Post-Crop Vignetting adjusts to accomodate any crop you may have made.  With a vignette created by your lens, there is nothing to fix if you have cropped off the corners, so Lens Corrections vignetting does not adjust for a crop.

The other reason to use Post-Crop Vignetting for creative vignetting is that you can do more with it.  You can come in further towards the center of the photo, darken and lighten more, feather more or less, and change the shape from rectangular to round.  Here is a post on achieving creative effects, like film borders, with post-crop vignetting.

Update: Moments after I posted this, I got a message from fellow blogger, Gene McCullagh, over at lightroomsecrets.com.  Coincidentally he was also writing a post on vignettes that day!   Do check it out this excellent post — he goes into more depth on what the sliders mean and how to get particular creative effects.  (As a great added bonus, he explains where the term vignette came from!)  While you’re there, do read more from Gene and subscribe to his feed — his articles are top notch.

Of course vignettes and much more are covered in depth in my Lightroom Fundamentals & Beyond: A Workshop on DVD.