Help me choose a lens.

[starletman]

I am looking for some more reach and have narrowed it down to these options.
I currently have a Sigma 70-200 F2.8.
So do i buy a 2x teleconverter for the 70-200,or buy one of the following.
Sigma 120-400
Sigma 150-500
Sigma 50-500
The budget will allow me to get a new 120-400 but the 50/150-500 would be used models.
Just after some opinions on what would work best on a D700 and D40.

 

[pete.rush]

What would you be photographing?

 

[starletman]

All sorts,some motorsport,planes birds anything that takes my fancy when i need some zoom.

 

[pete.rush]

The sigma 50/150-500mm are very good wildlife lenses. All 3 lenses I think have the same OS system. The 50-500mm is supposed to be the slightly better lens, some reports of being soft at 500mm others sharp at 500mm so depends on your copy, but conditions would effect your shots being a f6.3 lens even with the OS system. Reports are the 120-400mm is as good as the canon 100-400mm (can compare the Nikon 80-400 because it ain't great)

Depends whether you want 400mm @ f5.6 or 500mm @ f6.3