jerr3d has it right...If you're using the main cam then open it up to around the 200mm mark, and render either three quarters of your image on each side and then stitch the results up in 'shop, or try doing it in 3 parts...left, center, and right and then stitching those three togther.  Hopefully the overlaps should "even out" the distortion somewhat.**
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The liver is evil - It must be punished.