The canon st e2 speedlite transmitter is an integral part of the canon flash system on camera wireless transmitter for speedlites 420ex 430ex ii 550ex 580ex ii and macrolites mr 14ex and mr 24ex the canon st e2 is a signal transmitter which fits into the camera hot shoe allowing wireless variable ratio control of up to two groups of compatible canon speedlites uses a 2cr5 battery not included