After watching the movie, ask students to list as many variables as they can think of related to photography (you may wish to have them do this in pairs or small groups).
Discuss answers and list several such variables on the board (be sure to include shutter "speed"--length of time the shutter is open, the brightness of the scene being photographed, size of aperture (how large is the hole through which light enters the camera when the shutter is open), and how fast the object being photographed is moving.
Now ask, "Suppose you know that you need a certain amount of light to enter the camera in order to expose your film properly (or be collected on the digital imaging device being use in the camera). If you want that amount of light always to be the same, how do you think the shutter speed might be related to the aperture? How do you think the brightness of the scene would be related to the shutter speed?"
You can also ask students to consider how the shutter speed might be related to the rate of travel of the object being photographed (higher rate of travel requires shorter exposure time so that the object isn't blurred).
Explain to students that a relationship in which one variable depends on another variable is called a function, and that functions are very useful because you can determine the value of one variable just by knowing the value of some other variable.
You may wish to explain that in fact the purpose of algebra is to make it easier to precisely describe how variables are related to each other and to determine the value of one variable when the values of other variables are known.
Tell students they will be learning more about several different kinds of functions in this chapter.
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