The canon speedlite transmitter st e2 is compatible with type a eos cameras e ttl auto flash and type b eos cameras manual flash the st e2 is designed to control slaved canon 550 ex flashes enabling simultaneous flash use in professional settings the wireless controller has a flash ratio control allowing for several different ab ratios 18 to 81 in half step increments as well as slave confirmation via a test transmission button which sets off slave a followed by slave b at 164 output allows