This article appeared in the March/April 2002 issue of PT. To purchase this issue and receive this and other valuable articles in this issue, CLICK HERE: Ship within the U.S. | Ship outside the U.S.

Dragging the shutter…what in the world does that mean? Quite simply, it’s balancing the exposure when combining strobe and ambient light. For example, it’s taking a photo of someone outside at night with a portable flash, and also capturing the city lights in the background. Strobes put out a powerful amount of light. To balance the ambient light, we need to use a long shutter speed, hence “dragging the shutter.” There is much to know about this technique because it involves both color and exposure balance. Some cameras have a function called “slow-sync” that works with a dedicated flash, automatically reading and figuring the proper shutter speed. With a little understanding, you can effectively use this technique with any camera and flash, and not just dedicated or same system ones.

Strobe Light and Sync Speed
First, we must understand how strobes or flash work. A flash is normally a portable strobe light, whether built into the camera or a separate unit attached at the hot shoe or by a sync cord. A strobe is an instantaneous burst of light, a mini-explosion that creates a large amount of light. The flash is so fast, it’s actually “on” for less than 1/1,000-second. Flash duration depends on the amount of power used; normally varying from 1/500 to 1/2000-second. With some specialized studio strobes, like the Profoto, the flash duration can be as short as 1/12,000-second, which will freeze any action. When taking an exposure reading with a continuous light source (anything but a strobe), we use the shutter speed, or the length of time; and the aperture, or the size of the opening that allows the light in. Think of light as having the properties of water: the volume depends on the size of the pipe, as well as on how long the spigot is open. Since the strobe light is on for so short a time, the only control we have over exposure is aperture. Shutter speed doesn’t matter; whether the shutter is open for 1 second or 1/500-second, the light is only on for 1/1000-second. When taking exposure readings with a strobe meter, set the shutter speed to the sync speed to see what ƒ/stop to use.

What matters, however, is your camera’s sync speed. Most 35mm cameras have focal plane shutters that consist of two curtains—one that travels across the film plane opening it to light, and another that follows by covering up the film plane. If you use a fast shutter speed, say 1/500, the second curtain follows closely behind the first. Consequently, the entire piece of film is never all exposed to light at the same time.

When you use a flash, it’s activated when the first curtain reaches the opposite side. Sync speed is the fastest shutter speed you can use that when the first curtain reaches the other side and the flash goes off, the second curtain hasn’t yet begun to close. Most cameras sync at a speed of 1/60; others sync as fast as 1/250. (Leaf shutters sync at any speed because the flash fires when all the blades are fully retracted.) If you set the camera to a speed faster that your sync speed, the second curtain will partially cover the film when the flash fires, thus blocking the exposure, and resulting in photos with a black band at one end. How large the black band is depends on the shutter speed.

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