Took my son to Meet the Mickey session today at City Square mall. Turned out the photo turned out to be soft even when I am using Sigma 17-70mm @ 17mm f2.8. I suspect that flashlight must be needed. Can I ask if I need to buy a external flash like Yong Nuo or Nissin to take sharp photo? And I saw something about Nissin being manual? That means I need to adjust the exposure to compensate for the external flash as the camera does not know its existence?