Digital Photography

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 aperture value), 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.

aperture-priority, depth-of-field, Exposure

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