Going Semi-Automatic: Aperture-priority Autoexposure
Over the past week or so, I’ve been opining about how exploring advanced exposure modes can help you take more creative control over your photos. Today I want to introduce you to one of the two common semi-automatic modes, aperture-priority AE (autoexposure.) Represented on camera mode dials or menus by the letter A or Av (for aperturevalue), it offers you an easy way to manipulate depth-of-field, or the zone of sharp focus in a photograph.
Just a quick recap of aperture basics: The aperture is an adjustable hole in a diaphragm that sits behind the lens in your camera. Through the camera’s aperture setting, you can change the size of that hole, allowing more or less light into the camera. The size of the aperture is stated in f-numbers or, more commonly, f-stops, and is written as f/2, f/5.3, and so on. The lower the f-stop number, the larger the aperture, and the more light that’s allowed to into the camera.
But as I mentioned, changing the aperture size also impacts depth of field. A low f-stop produces a short 
depth of field, so your subject appears sharply focused but distant objects are blurry. A higher f-stop increases depth of field, so those distant objects appear more sharply focused. The photos here, from Digital Photography For Dummies, 6th Edition, offer an example. I took the first picture using an f-stop of f/3.4, and the second at f/11. Notice that the background is less sharply focused in the f/3.4 example than in the f/11 example.
Of course, as you allow more or less light into the camera, you have to adjust one of the two other exposure controls, shutter speed or ISO, to maintain the same exposure. And that’s the cool thing about aperture-priority autoexposure: The camera takes care of the needed shutter speed or ISO adjustment for you. .) All you do is decide how much depth of field you want and then dial in your preferred f-stop setting. (Whether the camera manipulates just shutter speed, ISO, or both depends on the camera, so check your manual.)
Do note a couple of fine points about aperture-priority AE:
- The range of aperture settings you can choose depends on your camera and lens. (Again, your manual will spell out the details.)
- In dim lighting, check to see what shutter speed the camera has selected after you set the f-stop. Be careful that the shutter speed doesn’t drop so low that movement of your subject or camera shake will blur the photo (anything below about 1/50 second takes you into the danger zone). For still subjects, use a tripod to avoid the problem. For moving subjects, you’ll either have to raise ISO or compromise on f-stop to get the shutter speed up.
- If the camera can’t select a shutter speed or ISO that will properly exposure your photo at your chosen f-stop, it should alert you in some way — blinking values in the viewfinder or monitor, for example. You can then choose to either go forward and take the picture or adjust the f-stop.
- Changing the aperture isn’t the only way to manipulate depth of field. You can also zoom in or move closer to your subject to reduce depth of field; zoom out or back away to increase it.
Okay, enough verbiage. Time to go get your camera and do some experimenting. Once you get a feel for aperture-priority mode, it’ll become your go-to mode for most still subjects, whether it’s a portrait, still life, product shot, landscape – you name it. Shoot each scene at several different aperture settings so that you can get a clearer idea of just how much playing with f-stops can affect your pictures.
Going Semi-Automatic: Shutter-priority Autoexposure
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.
Learn To see creatively By Bryan Peterson
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