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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.
Depth-of-field
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:
- use a smaller aperture (larger f-stop),
- 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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