Mastering Your Flash 101 – High Speed Sync
One of my favorite features of speedlites is High Speed Sync. This feature allows you to work with speedlites way past your camera’s normal sync speed (1/160 – 1/250 depending on camera). By shooting at very fast shutter speeds you can dramatically cut ambient light and can shoot what looks like night-time shots at high noon. Sounds incredible huh? Guess what, using it is as simple as pressing a single button.
Understanding The Shutter
First we need to look at how a typical camera shutter works. In the shutter mechanism are two “doors” or what are called curtains. When you press the shutter, the first curtain drops down out of the way, the sensor is exposed, and then the second curtain drops down to cover the sensor and stop the exposure. In this mode of operation, the flash goes off when the shutter is fully opened to get a good exposure. At slower shutter speeds, when you are trying to get more ambient light cooked onto the sensor, you can control if the flash goes off right when the first curtain is open or right before the second curtain closes. This can allow you to create interesting effects.
Once you get above your camera’s sync speed, usually around 1/250th of a second then the shutter starts to behave differently. At faster speeds the second curtain starts to close before the first curtain has fully opened causing a small gap between the curtains to move across the shutter. If you fire a flash only a portion of the sensor will be exposed during the duration of the flash bulb going off. This will cause banding in your image.
What is High Speed Sync?
Since a single flash burst will only expose a small portion of the sensor the solution is to have a continuous light output throughout the shutter cycle. Without speedlites this isn’t possible but we do have an interesting work-around. With High Speed Sync (HSS) the speedlite outputs a large number of short bursts while the shutter is in motion, around 50,000 bursts per second, emulating a constant light source. There are two downsides to HSS mode, the first is that you will lose some light output since the speedlite can’t put out full power when trying to spit out tens of thousands of bursts. The second downside is that you cannot freeze action with a longer shutter speed since the light output isn’t a very short burst, however, you can freeze just about anything by speeding that shutter up. This means the best use of high-speed sync is to light a subject that is heavily backlit by another light source such as the sun. Even in the brightest daylight can be taken to almost pitch black with a fast enough shutter speed. The challenge is to get enough light from your speedlight to get a good exposure on your subject.
Normal Flash Mode
Flash Bursting in High Speed Sync Mode
Enabling High Speed Sync
On the Canon speedlites its very simply to enable high speed sync. To get started, simply press the third button until the icon appears at the top of the speedlite’s LCD display. High Speed Sync is available in both eTTL mode and Manual mode. If High Speed Sync is not enabled, the fastest your camera’s shutter will be 1/250th of a second. Once you enable High Speed Sync you can set your shutter speed to any speed that your camera body will support. Most modern DSLRs can go up to 1/8000th second.
Why Do We Need Faster Shutter Speeds Anyway?
If 1/250th is fast enough to freeze most action, why would need to shoot faster than that or why would we need to use flash outdoors in the daylight? To begin with, if we are shooting people against a bright blue sky and we try to expose the scene for our subject, the blue in the sky will get completely washed out and turn white. On the other hand, if you crank the shutter speed fast enough to get the sky to stay nice and blue, the subject will be underexposed. The best way to solve this is to expose for the sky and then use flash to light the subject. While this sounds simple the problem is really bright conditions is that you may need a shutter speed significantly faster than your sync speed, sometimes well over 1/1000th of a second. Let’s take a look at a very typical outdoor, mid-day shooting situation.
In the first sample set the sun was almost directly overhead and subject was sitting in a gazebo so she was covered by shade. While the first shot is “ok” it certainly doesn’t stand out or have any real mood to it. The second image is unusable, and the third image has a real edgy tone to it due to the directional lighting that was done completely by flash since the settings and conditions were identical to the second image where the subject was completely dark.
In the second example set we set our subject completely unshaded and then started with an exposure metering of the sky and the knocked it down another two stops to knock it down as much as possible. Then, with the flash, we added the light back in with the flash.
The only issue was that the subject was basically looking into the direct sunlight, but we did want to prove the point that you could completely override the sunlight with a fast shutter speed. To take it a step further, we moved the light to the opposite side. Take note that the shadow side of the face was actually facing the sun but by cranking the shutter speed we virtually eliminated the effect the sun had on her face. We also changed the ISO from 400 to 100 to stop the overall lighting down by two stops without affecting the subject.
As you can see, we ended up with a great short lighted loop light. Could you do this with just natural light and reflectors, well not exactly. You could use a scrim over the model to cut down the sunlight and then bounce sunlight onto the far side of the face with a reflector. However, as you sped up the shutter to kill the background light, you would also be reducing the sunlight so it would be difficult, if not impossible to completely replicate. We can do it easily with flash because a speedlite at close range is going to provide more light than the sunlight and it is because we can create more light that we can do shots like this with flash.
Feel The Need for More Speed(lites)
Now let’s say you need to create a shot that appears to be taken at night, but your only opportunity to get the shot done is at high noon, the sun is blaring down and reflecting back up the ground. 1/100th at f/16 will give us a good exposure but if we need to knock it down significantly, without having enough depth of field to reach into the next county, we can easily find ourselves in a position where we need to be at 1/4000th or even faster to kill off the ambient light. Now usually we can get into the ball park with a single flash, but there is a really good reason why several companies have come out with dual, triple, and even quad speedlite holders. Remember, once we are in high speed sync, we can lose 2-2.5 stops of light, but if we add a second speedlite we double out light output. If one speedlite at full power (not really full because of HSS remember) isn’t quite enough, then two speedlites may be more than enough, as you add a third or fourth speedlite, you can usually run the speedlites are much less than full power which means faster recycle times and longer battery life. Running two speedlites each at 1/2 power is much better than 1 speedlite at full power. So three at 1/3 power each is better still. If you want to get really crazy, try twelve speedlites all connected together.
Hopefully you have now seen the power of using High Speed Sync to get shots that your normally wouldn’t be able to get. It does take some experimenting to get your settings dialed in for exactly what you are trying to achieve, but once you get the hang of it, it will totally open up your ability to be more creative.
|Camera:||Canon EOS 50D|
|Lighting:||Canon 580EX II|
|Triggers:||PocketWizard ControlTL System|
|Processing:||Adobe Lightroom 3.0|