Mark: Rented a big Canon 400mm for a while and found it too heavy for in-flight bird photography - doable, of course, and the shots were great, but my shoulders would *not* sustain the usage. This Sigma 50-500 was acclaimed as being relatively light - and sure enough, I was able to spend a couple of hours shooting at a wildlife sanctuary without feeling sore afterwards, and after the first hour of shooting I was reliably tracking medium sized birds (ducks, grackles, seabirds - *not* tree swallows) at full 500mm extension. For the photography I do, a tripod isn't useful, so weight matters - and I'm mostly working in daylight, so overall reach is more useful to me than aperture - within limits, of course, and stabilization does help too. I did find the autofocus a little "twitchy" with this lens on a Rebel XS, especially compared to my usual 55-250, when shooting small foreground objects (perched or in flight birds); finding a way to hold the lens *and* focus it *and* press the shutter release was challenging but by the end of the weekend I was starting to get the hang of it.