What lenses do you have already and why are you interested in IS? IS isn't designed for photographing moving objects, what are you intending to shoot with it?

The low end IS lens is the 28-135IS. A VERY good walk around lens.
Then there's the 70-200 2.8IS (the best of the best)
Then the 70-300IS. (eh, don't really like it)
then the 100-400IS (2nd only to the 70-200)

In my opinion, the mac-daddy of the 3 is the 70-200.

Canon lenses hold their value extremely well, a 2 or 3 year old excellent quality copy of any of those lenses will pretty much cost the same as new. You can check bhphotovideo.com, they have a used equipment section on their site, but nothing lasts long. You can also check out fredmiranda.com, he also has a buy/sell section.

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for a relatively shorter telephoto the IS 28-135IS is a great choice. Cheaper as compared to others and a good everyday lens.

For anything around 300mm you would need a monopod or a tripod anyways. Why not look at the Bigma then? (Sigma 50-500mm). It is big, heavy and great zoom and for the price it is awesome.

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I have a 28-135 is lens already, which seems to be (in my whole one week experience) a very good lens. I also have a 70-300 Canon lens, but it's impossible to use w/o camera shake at the long end. I took some zoomed in photos, and they're a bit blurry w/o a tripod. I figured the IS would allow me to not have to use the tripod.

I also have a 28-80 USM - for what I'm not sure.

Mostly, I want to take on-the-fly shots of the kids playing softball, stuff on the boat, and some work-related things. None of those are very convenient for carrying a tripod.

I guess also, given that the camera is 8 MP, I could take shots at 135mm, then zoom in and crop on Photoshop, and still have something left that is high enough resolution to include in a work product or to print a photo out?

If that's the case, maybe I'll do that.

I was thinking a longer IS, a lens with a lower f rating, and maybe something like a 50mm macro, and I should have more goodies than I can carry or will need.

Maybe I need to just try it out for awhile before I spend any more on it.

Thanks.

ps - thanks for the websites - I'll take a look at them. No bargains on this stuff, that's for sure.