A photograph is a captured moment in time - an event frozen for ever on a small screen or a piece of film or paper. In almost every case, something in the captures scene is moving at the time of the exposure. This movement causes a degree of blurring of the moving element which is dependent upon the speed and direction of movement and the shutter speed selected by the photographer. Clearly, if the shutter remains open for a longer period, the extent of the movement recorded increases. Where a very fast shutter speed is selected, and the exposure time is reduced to perhaps 1/500 second, most movement in everyday life is effectively frozen.

Blurring of an image can also result from movement of the camera itself. Human beings are unable to hold a camera perfectly still, and with care it is possible to observe camera movement while looking through the viewfinder. Try aligning the edge of a picture with the side of a building and watch how the building appears to wobble to and fro. In windy conditions it is inevitably even more difficult to achieve hand-held stability. The effect of camera movement is to blur the whole image, so it is more likely to ruin a picture than subject movement which blurs only the element of the image which changes position during the exposure.

The degree of camera shake, and the minimum shutter speed required to effectively eliminate it, are dependent to some extent on the individual photographer and the physical circumstances in which an image is captured. Some people can just hold a camera more steady than others. There is also an element of experience involved, because it is possible to improve the situation by learning how to hold a camera and how best to position oneself when taking a photograph. However, experience shows that it is unwise to hand-hold a camera when using shutter speeds slower than certain limits. These limits are dependent primarily on the focal length of the lens being used. When using a telephoto lens it is much more difficult to keep the image stationary than with a wide-angle lens.

The rule of thumb that has evolved over the years is that the slowest acceptable shutter speed for the purpose of avoiding hand-held camera shake is given by the reciprocal of the focal length of the lens in use. For example, with a standard 50mm lens the minimum recommended speed is 1/50 second, and when using a 135mm lens the minimum speed is 1/135 second (in practice 1/200 second).

The table below gives the practical recommended slowest shutter speeds for popular lens focal lengths.