This page sheds some light on compatible lenses, speedlites (external flashes), memory cards, remotes and other gear for the T3i.
Lenses
The T3i is compatible with Canon EF and EF-S lenses. It’s also compatible with Tamron, Sigma and Tokina lenses designed for Canon DSLR’s.
External Flashes (Speedlite’s)
The T3i is compatible with EX-series speedlite’s, including the 270EX II, 320EX, 430EX II and 580EX II. It also works with macro ring flashes, including the MR-14EX and the MT-24EX. Third party flashes can synchronize with the T3i at shutter speeds of 1/200 or slower, however, they won’t work when ‘Live View’ mode is enabled.
Memory Cards
The T3i can use SD, SDHC or SDXC memory cards. Check out my in-depth recommendation (see here) for the T3i which includes specific requirements for HD recording, the number of images you can expect to take, estimated movie recording times and other important info.
Batteries & Power
The T3i uses a rechargeable LP-E8 lithium-ion battery, charged with a LC-E8 battery charger. In use, I’ve been able to get around 440 to 550 shots per charge depending on how much flash is used. For added power, you can use the BG-E8 battery grip which doubles your shots per charge.
Remotes
The T3i is compatible with the wired RS-60E3 and the wireless RC-6 remotes. It’s also backwards compatible with the RC-1 and RC-5 remotes, however, the RC-5 can’t be used to take still photos while movie mode is engaged.
External Microphones
The T3i is compatible with external microphones equipped with a stereo mini plug (3.5mm diameter).