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Saturday, September 1, 2007

Seeing in Slo-Mo

How to get motion blurs that will add a new dimension to your photography

Long exposures can blur moving subjects and portions of scenes into fascinating forms, revealing flows of motion and form that can’t be seen in an image made with a short exposure. All you need is a slow shutter speed, a sturdy camera support and your imagination.

Getting Long Exposure Times

Most digital and film SLRs let you set shutter speeds down to 30 seconds. For longer exposures, you can use the B (bulb) setting. On B, the camera shutter opens when you press the shutter button and stays open as long as you keep the shutter button fully depressed. To prevent finger fatigue, and to keep you from accidentally jiggling the camera, use a locking cable release to hold the shutter button down during long exposures.

Some cameras have a T (time) setting rather than B. On T, the shutter opens when you fully depress the shutter button and stays open until you press the shutter button again, saving you the bother of holding the shutter button down. In bright light, you won’t be able to stop the lens down enough to use very long exposure times: at ISO 100, the exposure duration for a frontlit sunny scene at ƒ/22 would be 1/50 sec. But there’s a way to get long exposure times in bright light: the neutral-density filter.

Neutral-density filters reduce the amount of light transmitted to the film or image sensor, without otherwise altering the light. ND filters come in a variety of strengths. The strongest are Kodak’s gelatin No. 96 ND 4.00, which cuts the light by 13 1/3 stops (www.kodak.com), Hoya’s NDx400, which provides nine stops of ND (www.thkphoto.com) and Singh-Ray’s Vari-ND, which provides strengths from two to eight stops of ND as you rotate it (www.singh-ray.com). A polarizing filter also reduces the amount of light reaching the film or image sensor, without altering its color, but only by 1.3 stops or thereabouts—not strong enough for daylight long exposures.

Tripod Or Handheld?

Long-exposure shooting is best done with the camera mounted on a sturdy tripod, especially the subject blurs discussed here. A cable release or wireless electronic trigger will keep you from jiggling the camera as you press the shutter button, adding to steadiness.

If your camera/lens has a built-in stabilizer, you can do pan blurs, zoom blurs and flash blurs handheld. I’ve had good results down to 1/20 sec. with focal lengths in the 200-300mm range using stabilizers built into lenses and cameras. But effective subject blurs generally require exposure times considerably longer than that. And if the nonmoving portions of such scenes are blurred, the image will look like a mistake rather than a good photograph, so use a tripod. With some stabilized lenses and cameras, you should switch the stabilizer off when using a tripod; with others, you should activate it. Check the instruction manual for your gear.

The Subject Blur: Long Exposure, Stationary Camera

Choose a scene that contains both stationary and moving elements, such as a waterfall, breakers on a rocky beach, waves of grain or flowers undulating in the wind, blowing snow or rain, whirlpools and eddies or freeway traffic at night. Put the camera on a tripod and shoot at a slow shutter speed. The stationary portions of the scene will be recorded sharply, while the moving elements will blur.

How slow a shutter speed should you use? That depends on the subject, how fast it’s moving, how far it is from the camera, the lens focal length and your personal preferences. Experiment with a range of exposure times the first time you try blurs with a given subject, and you’ll quickly learn from examining the photos which shutter speeds you prefer for that subject. For the subjects just mentioned, try exposure times from 1/15 sec. to 30 sec., and see which resulting images you prefer.

You can use very long exposures to make unwanted people disappear from landscape and architectural shots.
If you expose for several seconds to a couple of minutes, people moving through the scene won't be in any one place long enough to register on the film or image sensor.

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