Auto FP/High Speed Sync can be a little confusing, and when I first wanted to explore it, there was a decent amount of information on it, but some was inaccurate or incomplete. I wanted to explain the technique in plain english. Come on inside and check out this fun way of capturing images that really pop!
A few months back I realized that my D90 supported Auto FP, and I’ve wanted to test it out ever since. The link in the previous sentence is a pretty long explanation, so here is the short explanation. Auto FP stands for Auto Focal Plane. But in reality it’s a fancy name for High Speed Sync. All DSLR’s have a maximum flash sync speed. Most are between 1/200 and 1/320. If you capture an image using your flash faster than these speeds, you’ll see a black bar across the photo, which is the shutter. On some mid to higher end Nikon DSLR’s, you can activate Auto FP, which allows you to raises the max flash sync speed to as fast as your cameras max shutter speed. My D90 goes to 1/4000, while many go up to 1/8000.
There are a few caveats though. The first is that instead of one quick burst of light like a normal flash, Auto FP actually fires off a bunch of less powerful flashes in a row. To the naked eye it looks like a continuous flash. The issue is this burst of flash is quite a bit less powerful than your normal flash. The faster the shutter speed, the less powerful the bursts are. At 1/4000, you’ll have to have the flash pretty close to your subject, and/or shoot wide open, the latter being one of the reasons many people use Auto FP. More on this below.
Another issue is that you must have a speed light that is compatible with Auto FP. Most of the Nikon speed lights are compatible, the SB600 up to the SB910 are, as well as a few 3rd party flashes. I originally was going to buy the Sigma EF-530 Super , but Amazon sent me the wrong flash and didn’t have the EF-530 in stock. The higher end Nissin flashes support Auto FP also. I ended up getting the SB-700 and am loving it.
You may be wondering why the hell anybody would need High Speed Sync. Just looking at the words, it seems as though it’s for freezing action at high shutter speeds using flash, when in fact it’s almost the opposite. As stated a few paragraphs up, the flash is a series of bursts that lasts the duration that the shutter is open, instead of traditional flash, which is fired either when the shutter first opens or when the shutter is closing (rear sync). Here is a real world example:
You are shooting outside at noon on Sunday. It’s pretty bright out and you want to add some soft, directional light on your subject. BAM! It’s HSS time! Why use HSS instead of just traditional flash? The first issue is that because your max flash sync speed is only 1/250, the ambient light is going to be totally blown out, it’s just too slow a shutter speed to shoot on a bright day. Now you could close down the aperture to say F16, or even F12. Here is where HSS comes into play:
You don’t want all of your image to be in focus, you want that nice bokeh your Uncle Bob told you about. You really want to shoot at F2 or 3. You can’t do this in bright daylight without blowing out the ambient. On top of that, if you close down the aperture too much, your flash is going to struggle to light your subject. Hence, High Speed Sync. It is admittedly of limited use, and you could under expose the background in post processing, but what fun is that? Here are a few images I took of my niece at about 3 pm a week or so back. We went for a Game of Thrones theme.
That image is directly to the right. I shot it at f5.6, ISO200 and 1/2000. My sister was holding the SB700 camera right, very close to Maia. We took a bunch like this that day. Another good effect is to just slightly underexpose the background and flash the subject. It’s a great way to really make the picture pop.
As you can see, I’m a big fan of high-speed sync. Although it has limited uses, when used correctly, it can produce some very cool images. Strobist has a ton of great articles on balancing ambient light and flash. I’ll be writing more articles on the subject as I learn more. Stay tuned!