Manfred, how did you go about dealing with vignetting via ACR?
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If it's a camera / lens combo that's in the ACR database then it can do it automatically, otherwise you need to do it manually on the lens correction tab (which is only available to full Photoshop users, not PSE users).
http://i39.tinypic.com/2w7py68.jpg
Colin, thank you for the response. I presume the lens correction in ACR is the same as in LR? I'll have to check whether I dealt with lens correction before exporting the original images from LR to Photoshop. I wasn't sure whether the vignetting was a product of the lens or of the stitching/merging process. (When I stitched the pano for the second time, I checked the vignetting removal option in Photoshop's merging program but still ended up with some mild vignetting.)
Colin beat me to the answer; I correct both the CA and lens distortion that way when I'm in a hurry. The database does not have all of my lens / camera combinations, but does have most of them. DxO Optics Pro does a better job, so I use it when I'm working on more critical stuff.