Thanks for your reply scarecrow - you have set me on the right track. I didn't know the term "barrel distortion". After further investigation it seems that photoshop [camera raw] corrects for barrel distortion but dcraw is a pure raw converter and does not do any corrections. The camera (in my case a GF1) also corrects for barrel distortion when producing jpgs. I guess that means that, for the time being, I will have to be content with Adobe Camera Raw doing the conversion.