The duration during which a camera's shutter is open and the film plane is exposed to light. These speeds are rated in terms of 'stops', each stop being being a duration twice as long as the previously slower one... Thus: 1 second, 1/2 of a second, 1/4th, 1/8th, 1/15th, 1/30th...etc...

Certain minimum shutter speeds are necessary in order to stop certain kinds of action from appearing as movement in a photo. For example:

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