Speaking of fireworks, I shot the NDP preview fireworks last Saturday. Practicing the black card technique (need more practice

).
Anyway, when I zoomed in the pic, I found that the light lines of the fireworks are not straight, more like curvy. I shot together side-by-side with a friend and he's using the same lens, but different body (D300s) and different aperature. His light lines are straight. I don't think it's my tripod because the lights from the building in the background are fine.
Anyone can share some light as to what may be the reason? :think:
PS. Will post some of the pics later for reference.