Alex, I think this depends on whether you are using full frame or not. On my D3 with this lens I got seagulls at the corners and edges. No lines. With my stars over the murray shot I used F3.5 got lines in the edges but no seagulls like at 2.8. I figure the extra real estate makes for the extra distortions at wide open. Are you using yours on a full frame or crop?
Sorry Phil for the hi-jack.
