Canon T3i vs Sony A57

no, not exactly these.  Just check the Canon web site for lenses.

but you don't have to match the focal lengths exactly to those of another brand. You simply choose the range that you are looking for.

recommended lenses:

canon 55-250 IS

Canon 70-300 IS

Canon 70-300 IS L

Canon 70-200 IS L  (f4 or f2.8)

Canon 100-400 L

not recommended: any of the Canon 75-300

Also apparently a nice, affordable lens:

Tamron 70-300 VC tele

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*** Life is short, time to zoom in *** ©