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Slow Sync Flash Photography

Sometimes your on-camera flash just doesn't perform the way you'd like. Foreground subjects are bright when background subjects are dark. Christmas lights are barely visible tiny spots of light. Understanding how your flash works will help you resolve these issues. There is one technique that will teach you all of the aspects of your flash: Slow Sync.

Flash Modes
First, you must understand that your camera has two main flash modes: Manual and TTL (Through The Lens). Your results will vary drastically depending on the flash mode you use. The default setting on your camera is TTL so we will start with using that mode. You can check your flash mode in your camera's menu setting under flash Control.

In order for your camera to calculate the correct exposure automatically it must measure the light bouncing off your subject before it takes the photo. This is easy when you are photographing ambient light because that light is always present. Flash however, is a momentary burst of light at the time you press the shutter release. So how does your camera measure the flash before the flash flashes? TTL flashes emit a pre-flash burst at a reduced power before the shutter opens. This pre-flash lights up your subject and bounces back to your camera and makes the appropriate calculations based on the amount of light bounced back. Once this calculation is made, the camera then opens the shutter and emits a burst of flash precisely calculated to expose your subject properly. This happens so fast that your eye cannot see that there are actually two flashes happening one right after the other.

Flash Pulse Width
So then, just what is the flash calculating to get that proper exposure? Think of the flash on your camera as a flashlight. The longer you keep the light on your subject, the more light is bouncing back into your camera. If you keep the flashlight on for 10 seconds, you will get twice as much light than if you had the flashlight on for 5 seconds. Of course, you would have to keep the shutter open for 10 seconds so that the image sensor can gather that light. The flash on your camera does the same thing only so fast that your eye can't see the difference. For a typical exposure your flash may be on for .25 milliseconds. If it needs twice the light then it may keep the flash on for .5 milliseconds. If it needs a lot of light it may keep the flash on for 1 millisecond. Each time we doubled the amount of light but the time is so quick that our eye cannot see a difference. This is known as the flash pulse width. In TTL mode, your flash is constantly adjusting the flash pulse width automatically so each photo is exposed properly. Well, maybe the foreground is exposed properly. Remember those concert photos where the back of the people's heads in front of you were exposed perfectly but the stage is very dark? Read on...

The Light Race
When the flash emits it's burst of light it races to the subject being photographed, bounces off and races back to the camera. The light that gets back first wins. What light will get back first? The light that bounces off the first object in the photo. Once this light reaches the camera, the light meter senses that enough light has returned and shuts down. Flash pulse width calculations are based on the first light that bounces back to the camera. This means that the closest subject will be exposed properly but anything further away will become darker and darker the further away it is.

Shutter speed and flash
Assuming your camera's flash mode is set to TTL or E-TTL we can now memorize two distinct rules:
  1. The Flash pulse width controls foreground exposure.
  2. The Shutter speed and aperture control the background (ambient light) exposure.
The fastest shutter speed of many cameras is about 1/8000 of a second (.125 milliseconds). It is extremely rare that you would ever use such a high shutter speed. Usually we are between 1/60 (16 milliseconds) and 1/1000 (1 millisecond). In automatic mode, your camera forces the shutter speed to 1/60 of a second the moment you turn your flash on. That means that your camera's shutter is letting light in for 16 milliseconds.

To better understand how flash and shutter speed react, take a look at this diagram. Shutter speeds longer than 1/1000 of a second have no affect on our flash. Unless you have a high-speed flash you cannot go above your camera's maximum flash sync speed of 1/60 to 1/250 of a second (read the flash tutorial). Therefore, we can pretty much select any shutter speed that we want and it will not change the effectiveness of our flash at all. As this diagram shows, flash does its job in less than 1 ms while ambient light is there all the time. We can take advantage of this lighting and use shutter speed to control the background exposure of our picture without affecting the foreground exposure.

Aperture and flash
Unlike shutter speed, the aperture affects all light entering the camera whether it is from your flash or ambient light. However, when in TTL mode, changing the aperture will not change the foreground exposure controlled by flash. Wait a minute. I just said that the aperture affects all light, including the light from the flash but that won't change the foreground exposure? The answer is simple. If you stop your aperture down so it lets less light into the camera the flash will simply put out a longer sync pulse to compensate. If you increase the aperture size the flash will put out a shorter sync pulse. In other words, with a large aperture the flash does not have to work as hard. But the foreground flash exposure will still be correct. So, unless a small aperture is really necessary, it is probably best to keep the aperture opened up and adjust the background exposure using shutter speed.

Slow Sync Photography
We learned above that there are two exposures when flash is used. The first is foreground exposure, being controlled by flash pulse width and the second is background exposure (ambient light) being controlled by shutter speed and aperture. It's almost like a double exposure photo. To give the photographer control of both exposures the camera needs to be in slow-sync mode. This can be accomplished simply by using shutter priority mode (S/Tv), aperture priority mode(A/Av) or full manual mode (M). When the camera is in Program (P) or Automatic mode the shutter speed will automatically go to 1/60th of a second once flash is turned on thus taking control away from the photographer. Some cameras actually have a slow-sync mode which basically disables the automatic 1/60th of a second shutter speed while in Program mode. Once you have control of the shutter speed you can start altering the background exposure by adjusting the shutter speed.


In each photo above the trumpet exposure remains the same. The background changes as the shutter speed changes.

Manual Flash
The biggest difference between TTL and manual flash is that in manual mode, the camera does not automatically adjust the flash pulse width. In manual mode the preflash is not necessary. Since the pulse width does not change, aperture will now have an affect on the foreground flash exposure. Stopping down the aperture will reduce the foreground exposure while increasing aperture size will increase foreground exposure. The aperture will also have an effect on the background exposure as well so be prepared to make adjustments.

Flash Compensation
We learned above that in TTL mode, the camera will adjust the foreground exposure by reading the light bounced back from the flash and adjusting the flash pulse width. This is done automatically but at times this automatic calculation may not be correct. There must be a method to control the flash exposure but still keep it automatic. This is called flash compensation. Flash compensation is an adjustment that allows the photographer to add or subject exposure from the automatic calculation determined by the camera. When flash is too harsh, flash compensation can be used to tone it down. Combining flash compensation and slow sync helps to blend the foreground with the background.

Flash EV:0
Flash EV:-1

First and Second Curtain
Slow shutter speed creates another interesting problem. If the flash burst is so fast and the shutter is open for a long time, when does the flash fire? Read the next tutorial "Slow Sync Effects".