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Electronic Guidebook for High-Speed Flash Photography

revised 12-00

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Appendix D. Photographic Basics


Selecting aperture (f-stop)


If you want to take good high-speed photographs consistently, one thing you need to know is how to select the aperture of your camera lens. On most lenses, the aperture dial has a sequence of numbers 22, 16, 11, 8, 5.6, 4, 2.8, 2, 1.4, 1.0. These numbers are called f-stops and are written f/22, f/16, etc. Some lens won't go as high as 22 and most won't go as low as 1.0. The f-stop is related to the size of the aperture in an inverse way; that is, the larger the f-stop, the smaller the aperture.


It's useful to know how many times more light one aperture lets in than another. For example, f/4 lets in twice as much light as f/5.6, f/5.6 lets in twice as much as f/8, and f/22 lets in half as much as f/16. Here's a problem showing how you might use this knowledge in flash photography.

Problem: Suppose you've determined that f/5.6 is the best f-stop to use for taking a high-speed photograph of a bursting balloon using Kodak Gold 400 film. Another person wants to take a photograph of the same subject, but she has Kodak Gold 100 film. What f-stop should she select?

Solution: Each film has an ISO rating which indicates its sensitivity to light. (Gold 400 has ISO 400 while Gold 100 has ISO 100.) ISO 400 film is four times as sensitive to light as ISO 100. This means that the ISO 100 film requires four times as much light to produce the same exposure as the ISO 400 film. The way to get four times the light is to open the aperture wider. f/4 would give twice as much light as f/5.6. Using f/2.8 would give another factor of two more light. Altogether, that would be 2x2 = 4 times as much light.




The aperture that you select for a photograph affects two things: 1) the exposure of the film and 2) the depth-of-field. Depth-of-field refers to the range of distances in front of and behind the subject that will be in acceptable focus in the photograph. There are two ways to increase depth-of-field when using a particular lens:

    1. use a smaller aperture (larger f-stop),
    2. move farther from the subject,

Suppose that you're photographing a balloon burst and you want the balloon to be in focus from front-to-back.You may not want to move too far from the balloon, because you want it to appear large in the photograph. That means you would need to use a smaller aperture. This may require you to use a more sensitive film, that is, one with a larger ISO rating. (See problem above.)



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