Using “Priority” Exposure Modes on Your dSLR

By Julie Adair King

In addition to regular autoexposure modes, where the dSLR camera sets both aperture and shutter speed, your camera may offer aperture-priority autoexposure or shutter-priority autoexposure.

On some cameras, aperture-priority autoexposure is abbreviated as A; on others, Av. The a stands for aperture; the v stands for value. Shutter-priority autoexposure is abbreviated by either the letter S or Tv, for time value. (Shutter speed determines the exposure time.) Oh, and if you see the letters AE, as you will if you read many photography magazines, it’s an abbreviation for autoexposure.

Whatever they’re labeled, these options offer more control while still giving you the benefit of the camera’s exposure brain. Here’s how they work:

  • Aperture-priority autoexposure: This setting gives you control over the aperture (f-stop). After you set the aperture, the camera selects the shutter speed necessary to correctly expose the image at that f-stop, taking into account the current ISO setting when making its decision.

By altering the aperture, you change depth of field — the range of sharp focus.

  • Shutter-priority autoexposure: In shutter-priority mode, you choose the shutter speed, and the camera selects the correct f-stop. This mode is good for times when your scene contains moving objects because shutter speed determines whether those objects appear blurry or are “frozen” in place.

Assuming that the ISO value doesn’t change, you theoretically should wind up with the same exposure no matter which aperture or shutter speed you choose, because as you adjust one value, the camera makes a corresponding change to the other value, right? Well, yes, sort of.

Remember that you’re working with a limited range of shutter speeds and apertures (your camera manual provides information on available settings). So, depending on the lighting conditions, the camera may not be able to properly compensate for the shutter speed or aperture that you choose.

Suppose that you’re shooting outside on a bright, sunny day. You take your first shot at an aperture of f/11, and the picture looks great. Then you shoot a second picture, this time choosing an aperture of f/4. The camera may not be able to set the shutter speed high enough to accommodate the larger aperture, which means an overexposed picture.

Here’s another example: Say that you’re trying to catch a tennis player in the act of smashing a ball during a gray, overcast day. You know that you need a high shutter speed to capture action, so you switch to shutter-priority mode and set the shutter speed to 1/500 second. But given the dim lighting, the camera can’t capture enough light even with the aperture open to its maximum setting, so your picture turns out too dark unless you increase the ISO setting, making the camera more sensitive to light.

These issues would arise even in manual exposure mode, however. If you don’t have enough light or if there’s too much light, you can’t dial in workable exposure settings, either. For this reason, you can rely on aperture- or shutter-priority modes for most shots. Letting the camera do half of the exposure heavy lifting frees you up to concentrate on other issues, including composition.