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Sunday, April 1, 2007

Digital Exposure Essentials

Getting the exposure right is at least as important when shooting digital as when shooting film

ƒ-Stops And Shutter Speeds

Each ƒ-stop number (ƒ/4, ƒ/5.6, ƒ/8, ƒ/11, etc.) of your camera represents a different diameter of aperture in the lens. The larger the ƒ-stop number, the smaller the opening in the lens, and vice versa. This may seem counterintuitive at first, but the reason for this is that the numbers are derived from a ratio between the focal length of a lens and the size of the aperture.

If you have a 50mm lens, for example, and the aperture is set at 25mm wide, then the ƒ-stop would be 50 divided by 25, or ƒ/2. The range of ƒ-stops will vary for different lenses, but the ƒ-numbers are all derived from this ratio.

Each ƒ-stop is used in concert with a shutter speed that will control how long light reaches the image sensor (1/25 sec., 1/60 sec., 1/250 sec., etc.) when you press the shutter release. The combination of these two settings—the size of the opening in the lens and how long the shutter stays open—is the exposure. Which shutter speed will best complement a particular ƒ-stop will depend on the lighting conditions and how you want to capture a scene.

If you want to freeze the motion of a bird in flight, a fast shutter speed is required and the ƒ-stop selected will have to complement this setting properly based on the available light. If you want to convey the movement of a bird with some slight blurring, a slower shutter speed is needed. With the right ƒ-stop and shutter speed, each image would have a proper exposure. It just depends what aesthetic qualities you want in your photograph, which includes depth of field.

Depth Of Field And ƒ-Stops

Digital Exposure Essentials

Remember, as the ƒ-stop number increases, the aperture gets smaller, giving your photograph increased depth of field. This is why landscape photographers frequently use ƒ/16 or ƒ/22; those settings will produce great depth of field and help emphasize distance. Everything in front of and beyond the focal point will be acceptably sharp, replicating how the human eye would view a scene.

Conversely, as the ƒ-stop number decreases, the aperture gets larger and the depth of field decreases. Everything in front of and beyond your focal point will be less and less in focus the larger the aperture. The amount also will depend on your distance from the focal point.

If you focus a wide-angle lens on something two feet away, an ƒ-stop of ƒ/2.8 will reduce the background to color and texture. Most of the detail beyond the focal point will be so out of focus that you won’t be able to tell what it is. Now, if you move farther away, say four feet, and focus on your subject again, an ƒ-stop of ƒ/2.8 simply will make the background less in focus than your subject. Whatever is behind your subject still will be visually discernable.

Depth of field also is affected by the focal length you select. Short focal lengths provide more depth of field than longer focal lengths. If you select an ƒ-stop of ƒ/11, for example, a focal length of 28mm is going to give you more depth than a focal length of 50mm.

Balancing Act

With every photograph you take, it’s always a balancing act of many different variables and compromises. You may need to experiment with different modes on your camera to find the right balance of ƒ-stop and shutter speed for the best possible exposure. And again, if your camera has the histogram feature, definitely use it. It doesn’t take a lot of time to study one, and it’s the best way to prevent poor exposures. You can make adjustments and refine your settings in the field so when you start editing images on your computer, you’ll have few problems, if any, to deal with.



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