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Action Shots: Faster than the Human Eye
A good camera can halt even the ultraquick motion of a football player midtackle or a ballet dancer as he leaps into the air. Learn what’s behind this magic with Groupon’s investigation into action shots.
To understand how a stationary photographer can capture a cheetah in midstride or the expression on its face as it dunks a basketball, it’s helpful to first consider how any camera works. When a picture is taken, the camera’s shutter opens and closes in front of the lens, letting in a precise amount of light for a set amount of time, depending on the exposure setting and the shutter speed. The lens lets in the light from anything that’s in front of it, which is then recorded on film or digital sensor.
This is not so different from how the eye and the brain process images. As on a movie camera, moving bodies register on the eye as a series of still shots that decay and are “refreshed” at imperceptibly small intervals, about 1/30th to 1/50th of a second. This can be considered analogous to the eye’s shutter speed. If the shutter speed of a camera is set around this range, it will capture motion in a way that looks natural to the human eye—that is, sharply if the image is a person ambling down the street, but perhaps more blurrily if it’s a tiny UFO speeding through an alley. If the shutter speed is slower, it will produce a blurred image, and, if it is much faster, it has the chance to capture instants that the eye can’t register clearly.
To catch fast-moving action, a shutter speed of at least 1/1000th of a second is usually required. The lighting, too, must be extraordinarily bright, since the quicker the shutter speed, the less light gets in; a photographer will widen the aperture to let in more light accordingly, and for long-distance shots, an electronic flash unit is required. There are a few other tricks in the action photographer’s bag. If you’re stuck with a slow shutter speed or dim lighting, you might have better luck aiming for the quick moment of stillness, or peak action, when, for instance, a figure skater stops being propelled upward and is about to sink back down. Another option is to set the camera itself in motion, smoothly panning in the direction of a bike racer, who will appear less blurry than the background.
As quick as the shutter speed may be, the photographer is eventually limited by the speed of the reflexes in the human hand. To overcome this barrier, systems have been invented that cause the subject to effectively take its own picture by crossing a triggering infrared beam or even making a loud sound.