Choosing a Shutter Speed to Stop Action

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The shutter speed you select for a given photo affects two key parts of your image: the overall exposure (when mated with the ISO and f/stop parameters) and the relative sharpness of your image in terms of how well subject or camera motion is stopped. A high shutter speed will freeze fast-moving action in front of your camera, and potentially negate any shakiness caused by camera movement. If a shutter speed is not high enough to stop this movement, elements in your image will be stretched and blurred. Creatively, you might actually want this blur in order to add a feeling of motion to your image. So, it's important to choose the right shutter speed to stop action when you want to freeze a moment in time or to allow your subject to "flow" when that's what you're looking for.

To maintain the most control over the amount/lack of blur in your photographs, you need to understand that components in your image are subject to this blurring to varying degrees. Indeed, it's possible to have one image with several subjects, each with a different amount of blur.

♦ Camera shake is an equal opportunity destroyer. As a practical matter, a shaky camera blurs all of your photograph to more or less the same degree. Objects closer to the camera may, in fact, appear to be blurrier than those located farther away, but the distinction is seldom important. Unless you have a creative purpose for camera shake, you'll want to use a high enough shutter speed to stop it, or steady your camera with a tripod, monopod, or other support. Of course, don't discount the possibility of using camera shake as a creative element, as in Figure 3.4.

♦ Motion across the frame is fastest. Motion that occurs in a plane that's parallel to the plane of the sensor will appear to move more quickly than motion that's headed toward or away from the camera. So, a racecar speeding from one side of the image to the other or a rocket taking off straight up will appear to be moving the fastest and will require the shortest shutter speed to freeze.

Figure 3.4 Camera shake during a one-second handheld exposure was used deliberately to add a pattern to these holiday lights.

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♦ Motion toward the camera is slowest. Motion coming toward the camera appears to move much more slowly. If that racecar or rocket is headed directly toward you, its motion won't seem nearly as fast (but it's still a good idea to get out of the way).

♦ "Slanted" movement is in between. If a subject starts out on one side of your frame, and approaches you while headed to the other side, it will display blur somewhere between the two extremes.

♦ Distance reduces apparent speed. Subjects that are closer to the camera blur more easily than subjects that are farther away, even though they're moving at the

Figure 3.5 Panning the camera in the direction of movement can stop action at a relatively slow shutter speed, while imparting a feeling of motion.

same absolute speed, because their motion across the camera frame is more rapid. A vehicle in the foreground might pass in front of the camera in a split second, while one hundreds of feet away may require three or four seconds to cross the frame.

♦ A moving camera emphasizes or compensates for subject motion. If you happen to be moving the camera in the same direction as a subject's motion (this is called panning), the relative speed of the subject will be less, and so will the blur. Should you move the camera in the other direction, the subject's motion on the frame will be relatively greater. An example of stopping action with panning is shown in Figure 3.5.

Figure 3.5 Panning the camera in the direction of movement can stop action at a relatively slow shutter speed, while imparting a feeling of motion.

The correct shutter speed will vary based on these factors, combined with the actual velocity of your subject. (That is, a tight end racing for a touchdown in an NFL game is very likely moving faster [and would require a faster shutter speed] than, say, a 45-year-old ex-jock with the same goal in a flag football game.) The actual speed you choose also varies with the amount of intentional blur you want to impart to your image, as in Figure 3.6.

For example, if you want to absolutely stop your subject in its tracks, you might need 1/1,000th to 1/2,000th second (or faster) for the speediest humans or speeding automobiles. You might apply 1/500th second to a galloping horse to allow a little blur in the steed's feet or mane. Shutter speeds as slow as 1/125th second can stop some kinds of action, particularly if you catch the movement at its peak, say, when a leaping basketball player reaches the top of his or her jump and unleashes the ball.

Figure 3.6 Because different elements in an image move at different speeds, you can select a shutter speed that will freeze some parts of your subject, while letting other parts blur in an interesting way.

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