Peripheral Vision

I was talking with someone the other Day and the old Chestnut of Peripheral Vision came up, I do not understand the thinking with many Wing Chun People that they say you defend yourself using Peripheral Vision, this is madness and will certainly fail if you are in Deep Poo. Peripheral Vision is very good and identifying even tiny movement in a wide Field of view, but once the movement is noticed you need to change back to focused Vision to see what was moving, this is what Eagles, Hawks and Army Snipers do, and many years ago when I was taught to shoot Game Birds this is what I was instructed to do, Peripheral Vision spots the movement and then Focused Vision Marks the Target. In Wing Chun I have heard even quite Senior Instructors say "Keep your Eyes on the Chest and pick up the attack with your Peripheral Vision", in a nice friendly session of 4 Corner Defence you may get away with this, but in a Street Encounter when someone pulls a Knife from out of their Jacket you will be going home in a Bag.