Originally posted by MRRiley High end zooms tend to be variable aperture... aka at 100mm they might be f4 but by the time you get to 300mm they are f8. A fixed focal length 300mm at f4 reduces a variable. when you go from 100mm/f4 at 1/1000s to 300mm in the same light you might have to drop to 1/125sec (since the loss of light is not simply linear). That 1/125sec is harder to hold still than if you could manage 1/1000 or even 1/500sec...
This is why constant aperture zooms are so expensive. The do offer the constant light gathering capability while providing the zoom capability...
my buddies canon 70-200 USM is a constant f2.8, at around 1500 (i dont remember how much he paid for his)
it makes wonderful pictures
Tokina also makes an 80-200 constant f2.8 unit for pentax, can be had for around 1000-1300 IIRC
what is the insentive for me to buy a 200mm fixed lens in light of other lenses on the market for a similar price? how much higher quality are we talking about here? is it really worth it?