# Picture This

Adapted from an article in "Math" magazine, December 6, 1991.

Taking pictures of celebrities is nothing new for photographer Kimberly Butler. Over the past 10 years, she's photographed everyone from journalist Barbara Walters to actor Alec Baldwin to former U.S. President George Bush. Her photos have appeared in "Time," "People," and "Sports Illustrated."

Math is nothing new for Kimberly, either. She uses it every day. Measurement and fractions help her make decisions that lead to perfect shots. "Math gives me a starting point," she said, "and then my eyes take over."

What happens when Kimberly takes a picture? The shutter of her camera opens, allowing light to enter the lens opening, or aperture (AP-er-chur). The light hits the film and forms the image that becomes the photograph.

When she takes a picture, Kimberly needs to know exactly how much light will reach the film. If the film doesn't receive enough light, Kimberly's photos will be underexposed and look too dark. If the film receives too much light, her photos will be overexposed and look too light.

Kimberly can control the amount of light that enters her camera in two ways. She can change the size of the aperture, and she can adjust the speed at which the shutter opens and closes. The smaller the aperture and the faster the shutter speed, the less light enters the camera. "It works the same way your eyes do," Kimberly explained. "When it's dark out, the pupils of your eyes open up to let in more light, just like the aperture of a camera."

The size of a camera's aperture is measured in units called f-stops. When the camera is set to a larger f-stop number, the aperture becomes smaller and lets in less light. Here are the f-stops found on most 35 millimeter (mm) cameras:

BIGGER LETS IN MORE LIGHT

 f5.6 f4
f2.8

SMALLER LETS IN LESS LIGHT

 f8 f11 f16

Shutter speeds are measured in fractions of a second. To open and close the shutter more quickly and let in less light, Kimberly sets the speed to a smaller fraction. Here are the typical shutter speeds on most 35 mm cameras.

SLOWER LETS IN MORE LIGHT

 1/60 1/30 1/4

FASTER LETS IN LESS LIGHT

 1/125 1/250 1/500 1/1,000

PICTURE PERFECT

To create the perfect picture, Kimberly must combine the right f-stop with the right shutter speed. How does she know which ones to use? That depends on what she's photographing. If she's taking an action shot, her shutter speed must be fast enough to freeze the action without making the picture look blurry. That means she has to open the camera's aperture by setting it to a smaller f-stop number. But if she uses a wide-open aperture, the background of her photo will be out of focus. So if Kimberly wants a completely focused shot, she must set the aperture to a larger f-stop number and use a slower shutter speed.

What else goes into taking great photos? "You have to be a people person," said Kimberly. "In a short time, you have to win the trust of someone so you can take the best possible shot." Her advice to future photographers: Get an education. "Being a photographer is also about being a good speaker and business person," she told "Math." "It's much more than taking good pictures!"

WHAT TO DO:

Take a "shot" at the photography problems below. Use the information about shutter speeds and f-stops to solve them.

1. Kimberly is about to photograph a tennis player racing for a ball at the net.

a. Should she choose an f-stop closer to f2.8 or f16?

b. Should she choose a shutter speed closer to 1/4 or 1/1,000?

2. Kimberly is shooting a model who's posing outdoors on a bright sunny day.

a. What f-stop do you think she should use?

b. What shutter speed do you think she should use?

c. Say Kimberly takes the shot using an f-stop of f16 and a shutter speed of 1/1,000. The photo turns out dark. What do you think she should do next time? Explain on a separate piece of paper.

3. Kimberly is taking a picture of former President Bush giving a speech outdoors on a cloudy day. Mrs. Bush is sitting behind the President, and Kimberly wants to be able to see her clearly in the shot. What f-stop and shutter speed should she use? Explain on separate paper.

4. Suppose Kimberly took the picture of a celebrity using an f-stop of f4 and a shutter speed of 1/60. Do you think she used bright or dim light? Explain on separate paper.

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