during which a camera
is open and the film plane
. 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: