I currently have the Canon 18-55mm kit lens that came with my Rebel XT. I also have a Tamron 70-300mm. I am looking at upgrading both of the lenses. I do a lot of airshows and landscapes, and quite a few pictures of people (not really portraits, though). I am looking at the Canon 100-400mm f/4.5-5.6L IS USM Autofocus Lens for my telephoto lens, and the Canon EF-S 17-85MM f4-5.6 IS USM as my wide angle and general-use lens. I do not really need anything less than 17mm, though. I also want better quality lenses than what I currently have. I also want image stabilization. Is the quality of the Canon 17-85mm better than my kit? Would you reccommend a different lens, or perhaps other than Canon? What lenses do you recommend? I want to slowly upgrade all of my gear, starting with lenses and then the a new camera, such as the 50D.
This question is so broad, I'll answer it just as broad.
a lens that you don't have.
Okay, seriously though, what do you find that your lenses are lacking? What ranges are they lacking in?
For instance, if you need a narrower depth of field, you'll need to buy "faster" glass, however if you just are looking for a step up in stopped down quality, you may be pleasantly surprised to find out that a lot of lenses clean up nicely stopped down.
What is your budget? I think that's the real question. I am not sure the second lens you listed is an uprade, but the first one is...given you can use it in good light. The Tamron 17-50 is a good lens GRANTED you get a sharp copy. The 70-200L is a superb lens (I still miss mine) However, the IQ of my Sigma 70-200 is pretty darn sweet. If you don't need the constant 2.8 then I would recommend the Canon 17-40L for sure!
OP from what you say your wants are I would go for the 100-400 as its probably the best airshow lens you will get but gan be a bit big and heavy at other times (much bigger and heavier than your tamron).
as for the shorter zoom I would avoid the 17-85 unless you can get it cheap and just want a general walk about lens , bad distortion at the short end and not the greatest IQ either. A cheaper alternative would be the Sigma 18-125 OS. if you want the quality and spend alot of time at the short end then go for 1 of the 17-50 range 2.8's Canon 17-55 has IS but is expensive. Tamron is very good and cheaper. if you shoot alot over 24mm then try the 24-105 IS.
As for the other question about the 17-85 yes its much better built than the kit lens
