In film and video cameras, the shutter is a plate located between the lens and the film stock or CCD chip. This plate has an opening that blocks and admits light at variable speeds according to the camera settings. The shutter speed is measured in fractions of second. 1/50 (0.02 second) is the standard shutter speed for film cameras, while for video cameras the standard is 1/60 (0.167 second).
Shutter speed determines the length that each frame is exposed to light, which, like the f-stop, affects the image overall exposure. The more a frame is exposed to light (when slow shutter speed is used) the brighter the image will be because light hits the each frame for a longer period of time. Conversely, the less a frame is exposed to light (when fast shutter speed is used) the darker the image will look.
While shutter speed is often non-adjustable in film cameras, video cameras offer a wide range of shutter speed options starting at 1/30 (slow, producing bright images) and going all the way to 1/4000 (fast, producing dark images).
Besides affecting exposure, shutter speed also determines the prominence of motion blur. Motion blur happens when an object in movement leaves a blur in the frame (or a series of frames). The blur is more noticeable during freeze frames and slow motions.
In the pictures below, the windflower is rotating at the same speed, but the motion blur varies in each picture because of the different shutter speed. Compare:
Note: on the images above other settings were adjusted so that exposure would be the same throughout.
Although shutter speed references the same feature in still photography, the results can be quite different when compared to video making. This happens because still cameras print the image in only one frame every time you press the shutter button. And the frame doesn’t change like in a film camera. Therefore, in still cameras, you can leave the shutter open for more than one second while letting the image be recorded throughout. Observe:
The photograph above was taken with the shutter open for 20 seconds, thus moving vehicles leave a trail. Also, note that the picture was captured at night, not day. But, as explained above, the longer the shutter speed, the brighter the image turns out; hence with a 20-second shutter speed, nighttime looks more like afternoon in a shaded street.