Originally Posted by randmc
Only you can answer that question, of course. []
The f/4 IS version is a little sharper than the f/4 non-IS version(especially away from the center), but the f/4 non-IS is still plenty sharp. Used on an XSi (where 'mid-frame' on the FF ISO 12233 crops is more like corner on 1.6x crop), the off-center differences are less. The 70-200mm f/4 non-IS is noticeably sharper than the 28-135mm, and obviously longer as well.
Partly it comes down to what sort of outdoor events you'll be shooting. IS reduces camera shake by enabling you to use a slower shutter speed - if you're shooting events that require fast shutter speeds to stop motion, IS is not that useful. Keep in mind that handholding a non-IS 200mm lens will require a shutter speed of ~1/320 (using the 1/focal length rule adjusted for 1.6x crop) to get a reasonable keeper rate. Depending on the lighting, that might mean higher ISOs than you'd otherwise want.
One short-term option is to buy a used non-IS version - I see them on my local Craigslist for $475-500 (sometimes a little lower, too). You'd probably be able to sell it for about that a few months down the line when you've saved the remaining $ for the f/4 IS version.
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