Shutter speed is one of several methods used to control the amount of light recorded by the camera’s digital sensor or film. It is also used to manipulate the visual effects of the final image beyond its luminosity. Images taken with a lower shutter speed invoke a visual sense of movement. Slower shutter speeds are often selected to suggest movement in a still photograph of a moving subject.
Excessively fast shutter speeds can cause a moving subject to appear unnaturally frozen. For instance, a running person may be caught with both feet in the air with all indications of movement lost in the frozen moment. When a slower shutter speed is selected, a longer time passes from the moment the shutter opens till the moment it closes. More time is available for movement in the subject to be recorded by the camera.
A slightly slower shutter speed will allow the photographer to introduce an element of a blur, either in the subject or if the camera is panned to follow a moving subject, the background is blurred while the subject remains sharp. When slower shutter speeds, in excess of about half a second, are used on running water, the photo will have a ghostly white appearance reminiscent of fog. This effect can be used in landscape photography.
In the image below, the object on the left was captured with a very fast shutter. The object on the right with a very slow shutter.
Also see: Aperture, Depth of Field and Exposure Value.