Light; much like water, will spill over if not controlled.
Often I use a Nikon SB-900 speed light with my Nikon D800e; and with it, I attach a honeycomb grid.
The Grid vectors / channels the light onto the subject and avoids spilling too far off
and depending on strobe to camera angle, ( I rarely mount my speed light onto camera )
will at times, reduce the lens flare caused by an off camera strobe.
Expoimaging makes such grids for speedlights.
Rick De Paiva