I own a Rebel T5. I recently got two studio strobes with built in optical slaves. My thought was to use the built in flash on the Canon to trigger the studio strobes. To that end I took a flash meter reading of the strobes, set the T5 on manual, keeping in mind the sync speed, dialed down the Canon flash output 3 stops and fired away. The strobes trigger but DO NOT SYNC. Red eye is turned off. I've tried other modes, nothing syncs. What am I missing? Help!