The brighter the light the less time the shutter needs to stay open in order to take in enough light for a picture. When the shutter speed is faster the lens eye stays open for a fraction of a second to capture that one moment before anything can move and avoid creating a blurry picture. So if the lighting is dark the shutter speed will need to stay open longer in order to take in enough light. When it is open longer more movement will cross its path and then everything is blurry.


