Originally posted by Eric Seavey Bigma (sigma 50-500mm) is a really nice lens. For about $1000 you can get a pentax mount. It is the equivalent of having two lenses in one.
I use my Bigma for airshows now...
How long you need on the long end really depends on the show.
My first year at the Binghamton airshow, the crowd was allowed very close to the runway at show center. I got great shots with a 50-200.
The second year, they moved the fence back a bit, and added a few obstructions at show center so it was actually the worst place to stand. I'm glad I had a Tamron 70-300 for that.
The third year, they moved the fence back even more and added a huge VIP tent between the main crowd and a significant part of the runway. Got less good shots than previous years despite having a Bigma. However, I would have gotten almost nothing w/o the Bigma.