Exposure and interrelationships of shutter, aperture and film speed

Shutter Speed is the setting that dictates for how long the shutter stays open to let light into the camera. Examples of typical shutter speeds include: 1, 2, 4, 8, 15, 30, 60, etc. Each number represents a fraction of a second. For example, 4 is 1/4 a second and 8 is 1/8 a second.

Aperture (Lens Opening) dictates the size of the shutter when it opens for exposure. It is measured in f stops. 22, 16, 11, 8, 5.6, etc. The lower the number, the larger the aperture.

Film speed is measured in American Standards Association units (ASA). Common speeds include 50, 100, 200, 400, ect. With ASA speeds, a doubling in the index number represents a film that needs half the exposure time (or one stop less) for a given subject. The film is twice as fast, so its speed rating is twice as high.

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The commonality between these three factors and the way that they are measured is that each step up or down the progression of measurement represents a doubling or halving of the light in the equation. A step up or down in shutter speed doubles or halves the amount of time light is allowed into the camera. A step up or down in stops doubles or halves the diameter of the aperture. A step up in film speed halves the amount of light necessary to expose the film.

From Wikipedia:

Four variables are available to the photographer to obtain the desired effect: lighting, film speed, f-number, and exposure time (shutter speed). The equation may be expressed as ratios, or, by taking the logarithm (base 2) of both sides, by addition. As a result, every increment of 1 is a doubling of exposure, known as a "stop". The f-number is proportional to the ratio between the lens focal length and aperture, which in turn is proportional to the lens area by the square root. Thus, a lens set to f/1.4 allows twice as much light to strike the focal plane as a lens set to f/2. Therefore, each increment of the square root of two (approximately 1.4) is also a stop, so lenses are typically marked in that progression: 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32, etc.