Which Canon Speedlites can I use with my Canon XTi?
The Canon EOS Rebel XTi features E-TTL II flash metering technology, and is most compatible with all current Canon EX-series Speedlites. From a weight/balance point-of-view the Canon Speedlite 430EX is probably the best match for your Rebel. If you need additional flash output, or would like the option of using external battery power you might want to check out the Canon 580EX Speedlite as well. These Speedlites work equally well with all other Canon EOS DSLRs. Earlier Canon Speedlites will also work, but depending on the specific model, may not take full advantage of the accuracy of the Canon E-TTL II flash technology found in the Rebel XTi.
