I am using Nikon SB600 speedlight in manual mode (with my Nikon D90) and shooting at F2.8 anything above 1/200 I am getting a black curtain covering half of the shot, gets wider as I go up in exposure. This is probably something basic and silly that I am missing? D90 is set to Manual mode with Auto FP on and highest available flash shutter speed available which is 1/60. I tried several curtain settings which all produce the same issue (rear, front and 2 others curtain sync modes available). Could anyone kindly shed some light on what is happening other than me being blonde (already known)?