Earlier this year, back in February, I was teaching at the Santa Fe Workshops. My class wanted me to demonstrate high speed sync for them. We stepped outside in the snow and I photographed a student with the chapel on campus in the background.
Nikon has this wonderful function called high speed sync (FP) that allow us to use our Nikon flashes above our native sync speed of 1/250th of a second. This first photograph show our subject photographed using our normal sync speed of 1/250th at f/7.1
Shooting with an aperture of f/7.1 provide too much DOF for my taste. Using high speed sync allows me the choice to shoot this photograph using a much shallower DOF.
By opening my lens to f/1.4 and raising my shutter speed to 1/5000th of a second, I was able to produce a much different looking image.
One of the downfalls of using high speed sync is that your flash looses a tremendous amount of power. I believe the power setting on the flash for the photo above was set to about 1/4 power. I was using a Nikon SB-800 shot through a Lastolite Tri-Grip just outside of the right frame of this photo. When I moved to high speed sync, With the flash at the same position, the power needed to be increased to full power. Here is the resulting photo showing a much shallower DOF.