Shooting in Program or Auto will, in most cases, produce a well-exposed image, but photography is much more than just pressing a few buttons or being a slave to a camera!

There are many occasions when the photographer needs to take charge and photographing movement is one where controlling the shutter-speed is necessary.

Shutter Priority (Time Value on some cameras) permits the photographer to choose the shutter-speed suitable for an application. When the shutter button is now depressed the camera selects the appropriate aperture for correct exposure. However, if the required aperture is outside the camera's range because the chosen shutter-speed is either too short or too long with a consequential risk of under or over exposure, it will give a warning signal.

A shutter-speed of extremely short duration freezes action, but a long shutter-speed will blur movement. A short shutter-speed is also useful for preventing camera shake with a handheld telephoto lens.