The amount of time a shutter remains open to allow light to strike the camera’s sensor is the camera’s “shutter speed.” A shorter time duration (aka “faster”) means more action can be stopped, or frozen. Conversely, slower shutter speeds (a.k.a. “longer”) allows action to blur. Under many circumstances, longer shutter speeds will necessitate smaller apertures, increasing depth of field (FIGS 1.6 and 1.7).
When a shutter speed actuation is made slower (by the shutter’s equivalent of a full stop), the amount of time that light plays on the chip is doubled. Changing the shutter speed in such a manner without correspondingly adjusting the aperture will result in an image overexposed by one stop.
When a shutter speed actuation is made faster (by the shutter’s equivalent of a full stop), the amount of time that light plays on the chip is halved. Changing the shutter speed in this manner without a corresponding change in f-stop will yield an image underexposed by one stop FIGS 1.8-1.10).
Canon cameras offer a range of shutter speeds from 30 seconds to 1/4000th of a second (on the Rebel) to a maximum of 1/8000 (on other models). All cameras have provision for Bulb exposures, where the shutter stays open as long as it’s depressed. Custom Function (C.Fn) 6 (on most cameras) permits you to choose intermittent shutter speed increments in % stops (camera default is V3 stop increments).
See also Aperture, Depth of Field, and Reciprocity.