Digital Photography

Does your camera offer shutter-priority autoexposure mode? Look on your exposure mode dial — it should be marked with the letters S or Tv. If you do have access to this mode, you’ve got a great tool for shooting scenes that contain movement, whether it’s your youngster lobbing tennis balls, a ferris wheel at the state fair, or even a flower blowing in the breeze.

In case you’re new to the concept of shutter speed, here’s a quick lesson: The shutter is a barrier inside the camera that prevents light from striking the image sensor (the part that turns light into a photograph). When you press the shutter button, the shutter opens briefly to let light through to the sensor. Shutter speed, measured in seconds, refers to how long the shutter remains open. In other words, shutter speed determines the exposure time – thus the abbreviation Tv (for time value) that’s used on some cameras to indicate shutter-priority autoexposure mode.

The shutter speed affects more than just how bright your photo is, though. It also determines whether any movement of your subject or the camera creates blur. Here’s the short story:

• Fast shutter speeds “freeze” motion; slow speeds blur it. Exactly how fast you need to go to freeze motion depends on the speed of your subject — the faster it’s moving, the higher the shutter speed you need. For example, check out the rose photo here, from my soon-to-be published tome, Nikon D90 For Dummies. Most people probably wouldn’t think “action shot” when photographing a flower, but if the wind is blowing, you need to see it as such. On the day I took this picture, a shutter speed of 1/40 second wasn’t fast enough to compensate for the motion of the flower, as you can see in the left example. I increased the shutter speed to 1/80 second to catch the blur-free version on the right. But if you were trying to shoot, say, an inline hockey player or a bird in flight, 1/80 second wouldn’t be nearly fast enough. So some experimentation is needed (although 1/1000 second should be fast enough for all but the speediest subjects).
• You can use very slow shutter speeds to your creative advantage, too. Experiment with using a very slow shutter speed — say, 1 second or more — to purposely blur motion for special effect. That’s how photographers produce those misty waterfall shots and ”trails of car lights” images you see in nighttime city scenes.
• But be careful about camera shake when using a slow shutter! Even the smallest movement of the camera during the exposure will blur the entire picture, not just moving objects. And the slower the shutter speed, the longer you need to keep the camera still. For most people, it’s tough to handhold a camera successfully at speeds much below 1/50 second, although the anti-shake features found on most new cameras do enable steady shooters to drop a little below that threshold. When in doubt, use a tripod or otherwise stabilize the camera to avoid the problem.

Now that you understand the relationship of shutter speed to blur, you can be more creative with your “moving pictures,” deciding whether you want to stop action or blur it. And with shutter-priority autoexposure, you can concentrate on shutter speed and rely on the camera to make the other necessary exposure calculations. You dial in the shutter speed you want, and the camera automatically takes care of the other two critical exposure settings, f-stop and ISO. (Check out the last several posts for details on the whole business of calculating exposure.)

Note just a few final details about shutter-priority mode:

• As you adjust the shutter speed, the camera has to adjust f-stop, ISO, or both to maintain the proper exposure. Remember that the f-stop setting  affects depth of field, or the range of sharp focus, while high ISO settings can produce the speckled defect known as noise. So after you select the shutter speed, verify that the f-stop or ISO the camera selected in turn won’t be a problem. For my rose photo, I was already at the lowest possible f-stop setting (f/6.3) for the lens I was using (meaning the aperture was wide open), so in order to use a faster shutter, I had no choice but to raise the ISO to 400. Fortunately, that’s well below the noise threshold on most cameras.
• The range of shutter speeds available to you depends on the camera; check your manual for specifics.
• With most cameras, using flash limits you to a top shutter speed in the 1/200 - 1/250 second range. However, if you can attach an external flash head to your camera, you may have more flexibility — it all depends on the camera and the flash.

action, Exposure, shutter, shutter-priority

Comments (1) | Add Yours   Share This