High-speed sync enables daylight flash at wide apertures by changing the way the flash fires.

According to the 1931 song, only “Mad dogs and Englishmen go out in the noon day sun.” Thanks to high-speed sync, flash photographers can now be added to the list. High-speed sync enables shutter speeds way beyond the norm for flash photography. It also opens up a new realm of creative opportunity.  Many shooters are intimidated by high-speed sync. In reality, it’s really easy to use if you have the right gear (say a Canon DSLR and a 580EX or a Nikon DLSR and an SB800).

Flash Photography Basics

I think of the amount of the flash as the duration of the flash. A speedlite ["speedlight" in Nikonese] firing at full power emits a longer burst of light than it does when firing at quarter power. When measured in a tiny slice of time, say a microsecond (one millionth of a second), the actual brightness coming out of the flash per microsecond is the same. At full power, the flash is just illuminating for more microseconds than it does at quarter power. [Note to uber-geeks: yes I know that it takes a few microseconds for the flash to reach full intensity and after peak intensity it falls off for a few microseconds, but let's not be too uber.] Looking at this backwards, if you want to freeze motion with flash, then use a high-powered strobe set to a low power setting.

The amount of flash can be controlled by the photographer, by the flash or by the camera. In manual mode, the photographer dials the amount of the flash up and down. [I do this often. To find out when and why, you'll have to wait for a future article. High-speed sync is way cooler than manual.] Some camera-mounted flash units have photo-sensors that will control the duration by measuring the amount of light coming back from the subject. [I think of this as "almost-matic" technology and never use it.] Today’s digital cameras have truly automatic technology in which the camera and the flash talk during the exposure. We refer to this as TTL – Through The Lens – flash. e-TTL in Canonese and i-TTL in Nikonese.

Sync speed is the fastest shutter speed that you can use during flash photography without “screwing up” the shot. If you use a faster shutter speed, a portion of your frame will not be illuminated by the flash. Ever have a flash photo with a dark band along one side? You shot faster than you sync speed. Of course, one shooter’s screw up is another shooter’s creative technique. Check out this video by David Ziser for an alternative look at purposefully  shooting faster than your sync speed.

The type of shutter in your camera (or lens) establishes the sync speed. View cameras (remember them?) and most medium-format cameras use lenses that have built-in leaf shutters with metal blades that open instantaneously from the center of the lens. Leaf shutters can synchronize with a flash at any shutter speed [technically, any shutter speed that is longer than the duration of the flash].

Single-Lens-Reflex cameras (film and digital) have shutters in which two curtains move across the focal plane. The interval between the curtains is the shutter speed. Essentially the exposure is a slit moving across the focal plane. For normal sync, SLRs must fire the flash after the first curtain is fully open and before the second curtain has started to close. Most DSLRs have sync speeds in the range of 1/125 to 1/250. At faster shutter speeds, there is no point when the entire sensor is exposed all at once – so normal sync is not possible.

Slow shutter speed = wide curtain gap, normal sync fine

Fast shutter speed = narrow curtain gap, must use high-speed sync

There are also cameras with electronic shutters that enable faster sync speeds (typically up to 1/500). Theoretically, electronic shutters can sync at any speed – but currently there are limitations caused by sensors over-heating… someday this will be really handy technology.

How High-Speed Sync Works

High-speed sync only works with dedicated TTL systems. The camera has to be able to talk to the flash. Further, you have to enable high-speed sync on the flash (Canon) or in the camera (Nikon).

With standard sync, the shutter has to be completely open when the flash fires. So, the camera fires the flash at the instant that the first curtain is fully open (“1st-curtain sync”) or at the instant just before the second curtain begins to close (2nd-curtain sync”). If you’ve ever seen a flash photograph taken at a really slow shutter speed where headlights of a car trail, that’s 2nd-curtain sync. [My cameras are always set to 2nd-curtain sync.]

With high-speed sync, the camera actually changes the way the flash fires. Rather than a single, strong burst, it tells the flash to send out an ultra-fast series of low-power, strobe pulses. Because the strobe pulses are so close together, the light appears to be continuous. So for the duration that the narrow curtain slit is traveling across the sensor, the flash is “always” on.

Canon’s Japan site has a useful diagram here. Fear not, it’s in English.

The upside of high-speed sync is that I can use virtually any shutter speed. The downside is that the output of the flash is greatly reduced. I often have to use several speedlites to get enough light when shooting at high-speed.

Freedom of Aperture With High-Speed Sync

High-speed sync gives me more creative freedom (aka: wider apertures). This is the main reason that I use it. Let’s say that I’ve been commissioned to shoot a portrait of a rising music star and that the only time she’s available is for 20 minutes at noon. Given that the sun is straight overhead, I’ll have to fill her face somehow. If I use a shiny reflector, she’ll squint and probably complain. So, I want to use a fill flash (or two) pushed through a white umbrella to soften the shadows.

With regular sync, the fastest I can shoot on my Canon 5D is 1/160. [Yes, I know the Canon specs say 1/200. But, in my part of the universe, it's really 1/160.] So, under the blazing sun, at 1/160, my widest aperture for a good exposure is f/13. That’s way too much depth-of-field for my portrait style. One option, would be to slap on my Singh-Ray Vari-ND filter and dial in several stops of neutral density. But, then I can’t see the magical expressions on my subject. The better choice is to turn on the high-speed sync on my 580EX (literally a quick button push). Now I can (almost) pick the aperture that I want to use. The reality was that there was too much sun. At 1/8000, the widest aperture I could get was f/5.6 – still, much better than f/13.

Taming The Sun With High-Speed Sync

Another benefit of high-speed sync is that you can overpower the sun with a small speedlite or two. In the photo above, you’ll note that the sky and background (basically, everything lit by the sun) is underexposed. Why? Two reasons. I wanted to make my subject the dominant element (aka: the brightest part of the picture). I also wanted to reduce the competition between the geometry of her arms and the geometry of the lattice. How? I set my overall exposure at -2 EV and my flash exposure at +2 EV. That’s about all the thought I put into it. I let the digital gnomes in my camera do the calculations.

In the following shot, you’ll see the underexposed daylight at left and the effect of the high-speed sync at right.

At left: Daylight only. At right: Canon flash at high-speed sync.

Setting Up High-Speed Sync

On Canon 580-series or 430-series speedlites, your flash must be set to ETTL mode. Then push the H-button until you see the H-icon on the screen. You are now in high-speed sync mode. Frankly, I keep high-speed sync on all the time. I shoot in aperture-priority (AV) mode 99% of the time – meaning that I’m almost always more concerned about controlling depth-of-field than I am about stopping motion. There’s no harm in leaving high-speed sync on. When I dial to an aperture that enables a shutter speed of 1/200 or slower, the camera automatically operates the flash at normal-sync mode – meaning that the full power of my speedlite is available.

On Canon Speedlites: Mode must be ETTL, then push the H-button. That's it. High-speed sync is activated.

On Nikon systems, it’s called Auto FP High-Speed Sync. As I understand it, you activate it in your camera rather than on the flash. Nikon shooters are encouraged to add comments relating to the specifics for their cameras.

Watch High-Speed Sync In Action

Watch this video of Joe McNally shooting high-speed sync in the sand dunes of Dubai. Thanks to the Strobist, David Hobby, for putting it together and launching it on the web.

Why did Joe use so many speedlights? You’ve already learned that high-speed sync greatly reduces the output. So if you have one speedlite and want to double the output (get another stop of light) you have to add a second speedlite. Then if you want another stop of light beyond that produced by two speedlites, you have to add two more speedlites. If you want third stop of additional light, you have to add four to the (one plus one plus two). So for three stops, you went from one light to eight. [Stay tuned... I've decided to round up as many 580EXs as I can and start firing them off en masse for a future article on this most-confusing concept.]

Wireless, High-Speed Sync with RadioPoppers

The opening image was shot using three 580EXs, a 430EX and a mess of RadioPoppers. One 580 was camera mounted with the Popper transmitter. It acted as the Master, but did not fire during the shot. The other three speedlites were mounted ala McNally on a C-stand at camera right and zoomed out to 105mm so that the light was concentrated along the length of the subject.

The Poppers provided eTTL control without too many hiccups.  The biggest issue was making sure that the fiber optic was placed exactly over the sensor. Not a biggie after we figured it out. Using the Poppers enabled me to forget about maintaining a line-of-sight between my Master and Slave units. I could work with my subject without concern for where the rack of speedlites stood. In short, I think RadioPoppers are great. New technology, yes. Rather expensive, yes. A bit buggie, yes. Able to do something that no other gadget can, YES. More on the Poppers to come soon.

RadioPoppers + Canon 580EXs = Wireless, High-Speed Sync