Hi I am still a relative new comer to photographer and have just gotten a 5D MKIII. I like to use 2nd curtain sync flash on some shots but the option appears to be greyed out on the menu of my 5D. I was previously using a 7D and was able to use 2nd curtain flash (both with the onboard flash and external speedlites) but it would ‘appear’ that I would need a Canon speedlite now. The 2 x speedlites I have are Yongnuo. I have triggers. Is there any way around this or have I no option but to buy a Canon brand one? Any advice would be welcome thanks!