Originally posted 2013-04-12 19:30:34.
Shutter speed is one of four interdependent parameters that are central to photography. These are : Aperture, Shutter Speed, subject brightness, and film or sensor sensitivity. IF any ONE of these is changed then AT LEAST one other must also change to maintain the same exposure. The most common example of this is that if aperture is altered, then shutter speed must also be altered, reciprocally. So if aperture admits less light, then shutter speed must admit more.
We have already discussed aperture, and seen how the f-stops marked on the aperture ring are calculated and what they are. To recap, the focal length divided by the aperture gives the f-stop, so a 50mm lens with a 25mm aperture is f2, and the they progress f2, f2.8, f4, f5.6, f8 and so on. Each of these allows half as much light to pass as the f-stop before, counting from low to high.
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