Im using Nikon D800E with my 24-70 2.8 lens. Problem hapens in low light situation using Yn equipment said above. I shoot at f5.6 and pics arent that sharp, but when i put my sb910 it s all good even on f2.8. Situation is a little better at f8 but it s too much for me. I m using YN's AF assist red grid and i allways get focus confirmation. Anyone had similar issue?
This is probably motion blur, your shutter speed is too long. Try opening it to F2.8. It is very strange that it gets better at F8 though, could it be a lucky shot?