Apart from exposing the film, the shutter also controls the appearance of motion. A fast shutter speed "freezes" action while a slower shutter speed allows movements to be blurred. One of the most basic camera-handling skills is to judge the slowest speed necessary to prevent or permit blur. Setting a speed that is faster than necessary requires an over-wide aperture to compensate.
What determines the stopping power of a given shutter speed is how fast the subject appears to move across the picture frame. With a normal lens, the image of a car moving across the field of view at 50mph (80 kph) will flash rapidly through the viewfinder if it passes close to the camera, but may take many seconds at a much greater distance. Similarly, a change to a wide-angle lens will reduce the image size of a moving object, so the object's motion will be relatively slower in the frame.
Stopping all motion is often assumed to be the ideal technique, and while it certainly gives the clearest image, it may not always be the most interesting or effective treatment. A slow shutter speed captures movement as a blur, and managed properly, this can give much more of a sense of motion. Our appreciation of movement in everyday life comes from observing continuous sequences, and the still photograph that freezes one moment of this sequence can sometimes look static and odd. It is important to know the pictorial effect of various degrees of blurring, so that you can choose the result that best suits the situation.
