Lightroom 3 has Lens Correction Profiles that correct for distortion, chromatic aberration and vignetting based on your lens model. Almost all of the profiles are for raw images only. For JPEG images, there are only a handful of profiles. However, I found out that you can convert the Lens Correction Profile for a raw file into one for a JPEG file.
The shot above was taken with the Tamron 17-50 VC for Nikon, an otherwise good lens that unfortunately suffers from noticeable distortion. I wasn't paying enough attention when I took the shot and had captured the image in JPEG instead of raw. Lightroom has a profile for the Tamron 17-50 VC for Nikon but only for raw images. I converted the raw profile into a JPEG profile and was able to apply it to correct the image. Hopefully the mouseover works on your browser (it does on my Firefox browser...):
How to convert a raw lens profile into a JPEG lens profile:
1. Find the lens profile to be converted. By default, this is C:\ProgramData\Adobe\CameraRaw\LensProfiles\1.0 and the specific subdirectory for the lens brand and camera version. For my lens it was in the Tamron subdirectory, and Nikon sub-subdirectory.
2. Open the .lcp file using any text editor (such as Notepad) and immediately save it as a new file (be sure to keep the .lcp extension). Out of caution, I first saved the new file into the desktop. I renamed the file to specify it is for JPEG, to help me differentiate it from the original lens profile.
3. Use Find and Replace to replace all tags like this
and replace it with this
Note: I used Replace All to replace the tags throughout the file.
4. Save the new file back into the correct subdirectory.
5. Restart or launch Lightroom 3. You should now find the new profile.
Disclaimers: a lens profile made for raw images won't work perfectly with a JPEG image. However, as shown by the demo images above, it certainly helps a lot, particularly with distortion. As mentioned in the forum thread I referenced, the vignetting correction may be excessive (in my case it wasn't). If so, reduce the correction and save the corrected profile as the new default.