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Monday, March 4, 2013

Understanding Aperture Priority


Don't just “take a picture,” “make a picture.”

Click Images To Enlarge This Article Features Photo Zoom

Don't just "take a picture," "make a picture." Take charge of the settings on your camera so the image turns out the way you want. Don't let the camera make the decision. While the PROGRAM mode provides a perfect exposure ninety eight percent of the time, it doesn't know that in situation A it's important that the depth of field is shallow so your subject stands out from the background. It's also obvious that in situation B, it's more important that the depth of field extends from the most foreground part of the composition and goes out to infinity. While the computers in today's cameras are amazing and let you capture a great image, the manufacturers still haven't figured out how to interface PROGRAM mode with your brain so the final image turns out the way you want. This being the case, it's time to take charge of your settings to wind up with an end result of what you envisioned when you pressed the shutter. This week's Tip focuses on Aperture Priority mode. Another will focus on Shutter Priority.

Aperture Priority: The "A" setting on your camera gives priority to the aperture. Based on the f-stop at which it's set, the camera chooses a corresponding shutter speed to provide good exposure. The given aperture determines the amount of depth of field in the photo. The lower in value the number, for instance, f/2.8 or f/4, the less depth of field. The higher in value the number, for instance, f/16 or f/22, the more depth of field. The more depth of field in an image, the greater the range of focus from the foreground to the background. Knowing how to control this range of focus is important when photographing subjects such as landscapes, people, animals, cityscapes, and macro shots. For instance, in a landscape, the guideline is to have everything in focus from the closest point in the foreground out to the farthest point in the distance. Use f/16 or f/22 to achieve this. Conversely, when photographing people, the guideline is to have the area both in front and in back of the person out of focus so the viewer's attention is drawn to the subject. Open the lens to the widest aperture to achieve this: f/2.8 / f/4 / f/5.6.

The amount of depth of field that can be achieved is dependent upon a number of variables that fall into place. When the lens is set to f/16 or f/22, the opening through which light passes is small. The result is not a lot of light reaches the sensor. This means the corresponding shutter speed to produce a proper exposure will be slow and necessitates the use of a tripod. It also means that if the subject is moving or if the wind is blowing a delicate subject, motion may be recorded. When all the variables fit together it's great. When they don't, a compromise between a desired aperture and the corresponding shutter speed must be made.

To show some real life examples, study the three photos that accompany this article. In the two images of the flowers, the first was shot at f/22, the second at f/4. The focal length for both was 180mm. The one shot at f/22 has a much greater range of focus. The corresponding shutter speed was 1/8 of a second. I had the camera mounted on a tripod and there was no wind. The flower made at f/4 had a corresponding shutter speed of 1/250th. The difference between the two is obvious. The f/22 photo provided a lot of depth of field but clutter and distractions dominate the background. The flower shot at f/4 produced a more pleasing representation.

As I stated above, the guideline for shooting landscapes is to maximize the depth of field and for portraits to minimize it. In the seascape, everything from the foreground rock with the sea star to the background sea stacks and sky are in sharp focus. The aperture at which it was photographed was f/22. The camera was on a tripod. If I tried to handhold it, the image would be soft because of the long shutter speed that was needed to get the proper exposure.

The important concept to glean from all of the above is to be able to predict the end result. Learn how a chosen aperture, its corresponding shutter speed, and focal length interact. Based on the required amount of depth of field you need in the photo, learn what needs to be done in order to achieve it. Set your camera to aperture priority and take charge of the end result.


Visit www.russburdenphotography.com

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