Reciprocal


Reciprocal is a math term that simply means an upside down fraction.  In photography, it usually refers to the rule of hand holding a lens.  The shutter speed should not be slower than the reciprocal of the focal length.

This means if you have a 300mm lens, you should not shoot at slower than 1/300s (1/300 is the reciprocal of 300/1).

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