Quantity of light

The quantity of light entering the camera is mitigated primarily by the F-Stop setting.

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The F-stop setting is determined by dividing focal length by Lens Opening diameter (aperture).
That means a 50mm lens with a 25mm lens opening = f2
A 100mm lens with 25mm lens opening = f4
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The focal length is the distance from the lens to the film plane and the lens diameter is the size of the circle allowing light into the camera. In most cameras the focal length is a set number so the lens diameter is what you are primarily affecting by twisting the stop ring on your camera.

The higher an f-stop is the less light will be allowed into the camera. *Remember This*

A certain progression of stops is mentioned in both the Horenstein reading and in the lectures:

These are important because each step in the progression represents a doubling of the light allowed into the camera.

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For an extremely detailed explaination of this topic see https://courses.utexas.edu/@@46B27A0E317E600BE5F8EFE5C940B423/courses/1/2006_spring_07380_RTF_318/content/_549523_1/docufax_20050807_142746.pdf or http://en.wikipedia.org/wiki/F_stop very detailed Wikipedia article.