This is something that new photographers seem to have a hard time understanding. High Speed Sync (HSS). To understand HSS, you must first understand how regular flash exposure works. In the most basic explanation, your speed light will throw light on your subject when the shutter opens. For most DLSRs, you cannot exceed a shutter of 1/250 (Native Sync Speed). If you do, you will see big black nothing-ness as the shutter closes during the flash cycle. Ok, so you are in bright sun. What are your options?
In this first photo, I did what you “typically” will do for a more dramatic sky. I underexposed the background first. Look at the sun. It is so bright. At native sync speed on my D3, I was at 1/250, ISO 200, and f/20. I then used my speed light manually to throw 1/1 (or full power) flash lighting on the subject. The result is nice, but the depth of field is really big. There is no dramatic bokeh or blur. The trees are all in focus. I don’t want that.
What if I wanted to shoot at f/2.8 in this situation? No way. Impossible. Well, it’s not. This is where High Speed Sync enters the game. Nikon calls it “Auto FP.” I want to create a more pleasing blur to the background. However, we have already established the fact that I need to be at f/20 for proper exposure. Correct? Yes and no. Using HSS, you tell the speed light that you really want to shoot at f/2.8.
The speed light says, “Well, ok. But here is the deal. I will do it, but I will only give you a small amount of light spread out over time because you are making me work so hard!”
The speed light will continue to throw small bursts of light while the shutter is opening and closing. The end result is that you can shoot at f/2.8 using a shutter speed way beyond f/250. In the case below, I was at 1/8000 second at f/2.8 using my Nikon SB-900 speed light.
The trade-off is that you lose power. So, if you are 20 feet away using a speed light you cannot really be so greedy and shoot into the sun with flash fill (or shoot at 12pm on a sunny summer day). However, it is a really useful flash technique to create more pleasing images in the presence of harsh light. I would use this for a bride and groom portrait at 10 feet away. For large groups, I would find a shaded area or use a much more powerful studio strobe with at least 320 watt-seconds if I was forced to shoot in the sun.
Make sure you read up on Front Curtain Sync and Rear Curtain Sync to truly understand the concept of how the flash interacts with the camera’s shutter.