f/3.8 at 1/8000 sec at ISO 100, 4:40 PM EST, SB-800 camera left, hand held in a Lumiquest SBIII, triggered with the pop-up flash, via Nikon's CLS system
Sounds like a mouth-full doesn't it? Sound like a really technical term? Well don't let the terminology fool you because its really a simple function to utilize.
...btw, the FP stands for "Focal Plane"
Shooting above your camera's native sync speed with off camera flash can be a really useful technique. Mostly this technique becomes really valuable when shooting outdoors, on bright sunny days and when you want to use a shallow depth of field.
Normally, when shooting outdoors in very sunny conditions, your settings are going to be around 1/250 sec shutter speed, f/16 aperture at ISO 200 to get a "properly" exposed image - when using strobes. Why? Well, because when using strobes/flashes synced to your camera, your camera is controlled by its flash sync speed. That sync speed is the fastest your shutter curtains can travel across your sensor while still picking up the burst of light from your flash. If you go over your sync speed, the shutter curtains travel faster than the light hitting the sensor and you will see dark bands starting to appear on the photos. That's a really simple way of stating a more technical reason, but hopefully you get the idea.
There are many other ways to "cheat the sync speed" but since I shoot Nikon, I thought I would discuss briefly, Nikon's Auto FP High Speed Sync mode. This method is really easy to use, provided that you have two required items. The first is obvious, you will need a camera with this feature built in. The second thing you will need is a Nikon flash that supports this feature. Fortunately, all of Nikon's latest flashes (SB-600, SB-700, SB-800 and SB-900) will support this.
In camera, you simply go to your "Custom Setting Menu", then down to "e Bracketing/flash", then to "e1 Flash sync speed" and choose the fastest sync speed option you have available, which on my camera is 1/320 s (Auto FP). I use a D300 but the location and settings should be similar on most Nikon cameras that have this ability, but check your user manual if you're not certain.
Once you have this enabled in your camera, you can just leave it on all the time. It will not effect your camera in any adverse way by leaving it set there, and what's more important, you won't get frustrated when you think you have it turned on (and its actually not), but it doesn't seem to be working, so ...just tune it in and "rip the knob off".
If you're using your pop-up flash as the triggering method for your off camera flash, then its important to make sure that you have the pop-up flash's output mode set to "--" because the pop-up flash cannot contribute any more light than is necessary to trigger the off camera flash when acting as the "commander" in this sync mode. So if you're running in to problems with this method, that's always one of the first things to check.
Now the downside to this method is, loss of power. In order to get your flash to fire above your normal sync speed, the flash has to pulse shots of light as opposed to one burst. These pulses of light are more under powered than a comparative burst of power. Also, using the Nikon CLS system, you have to make sure that the eye on the flash can see the signal from your pop-up flash, or on camera flash. This can give you fits, but with a little planning and forethought, you can make this work.
Now back to why would you want to use Auto FP High Speed Sync. For one (and probably most important to me) it allows you to shoot with much larger apertures giving you a very shallow depth of field which gives you that ooey-gooey out of focus background that really makes your subjects stand out. The images of my kids show a good example of this. Both of these images were taken at roughly 3pm EST with an off camera SB-800 in a shoot thru umbrella, triggered with another on camera SB-800 acting as the master.
Another reason to use AFPHSS may be to freeze action. Granted that flashes and strobes can freeze some action, however its not perfect for really fast movements. You get that "ghosting" effect where the subject in motion seems to have a trail or blur around them. That's because the flash freezes the subject for only a very short duration, maybe around 1/10,000 of a second, so shooting at faster shutter speeds, helps to eliminate that "ghosting" effect by effectively capturing the image faster than the action is occurring.
The last reason (that I can think of) to use AFPHSS is to give the illusion that a scene is darker than it really is like in the portrait at the top of this post. Its entirely possible to make a shot taken at noon, look like it was taken at dusk. By stopping down your aperture to say f/11 or smaller and using a faster shutter speed, like 1/4000 sec, you can literally turn day to night, and by throwing a little flash in there, you can have a really cool, and dramatic image. Don't believe me? Go try it, but be warned, you WILL be addicted. :)
I've had a few folks recently ask me questions about Auto FP High Speed Sync so I thought I would do my best to sum it up in a blog post. If I left anything out, or if anyone has any questions, please sound off in the comments and I'll do my best to field them... but most likely after I get the yard mowed. ;)