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huge mass of people writhing against each other. unless you buy your way into VIP areas, you're going to have people's elbows up your nose. if you must go, bring an unobtrusive, wide-angle lens that won't catch on things. close-up, PJ-ish shots are pretty much what you're in for unless you can score a room overlooking one of the main piazzas or want to hang out away from any of the action. having drinks spilled on your gear and fingers on your front elements are also possibilities. that's not to say that carnival isn't fun - it's just huge and hugely-touristic now.
if i were to go again, i'd take an NEX-C3 (or 5N) + 16/2.8.
just me 2 cents. cheers, and have a blast if you do go.