NB: This series of posts will convey my understanding of photographic exposures. While I hope it serves a useful purpose, I by no means describe this series as a be-all, end-all resource on exposure. As ever, if you feel I have made a factual error, please correct me.
Exposure is a combination of how long and how much light an imager (digital sensor or film) is exposed to by the lens and shutter. ISO/ASA (and less commonly nowadays DIN) is the standardised measure of an imager’s light sensitivity (i.e. sensitivity dictates how much light and how long for). In Part Two of this series, I shall focus on “how long” (shutter speed).
Shutter speed is the length of time the shutter exposes the imager to light. Shutters are generally leaf shutters (several metal leaves which open and close in circular fashion) or focal plane shutters (curtains of either metal or rubberised fabric, which travel vertically or horizontally). As a rule, leaf shutters provide less shutter-induced vibration than focal plane shutters, though focal plane shutters frequently travel at higher speeds than leaf shutters.
Shutter speed varies from fractions of a second (e.g. 1/4000) up to longer periods, which may even cover minutes, or hours. Many cameras have a maximum shutter speed of 30 or 60 seconds, but may offer a “bulb” mode, which allows the shutter to be held open for longer. Longer speeds tend to require the use of a tripod to prevent the camera from being moved during the exposure. The “sync speed”, is the fastest speed at which a flash gun can be synchronised. Leaf shutters can synchronise at their highest speed (up to 1/1000 second in some cases) or slower, whilst focal plane shutters generally synchronise at 1/125-1/250 second (depending on the camera) or slower. I will discuss this in later posts on flash photography.
Shutter speed controls the perception of movement. Longer (slower) shutter speeds, such as the 5.2 second speed in Figure One, cause movement to blur. This may be used for, amongst other things, blurring moving water. Under low natural light conditions, this also exposes the sensor to light for longer, which may be useful for landscapes and architecture where extra lighting (e.g. flash) is impractical or impossible.
Conversely, shorter (faster) shutter speeds freeze motion. The 1/800 second shutter speed in Figure Two freezes the motion of the waves at sea; you can readily differentiate between each wave forming out to sea, and clearly see the outline of the waves breaking on shore. Short shutter speeds frequently find use in sports for freezing rapid movements. The same effect is useful when photographing flowers on a windy day.
There is no hard and fast rule about using short shutter speeds for moving subjects. Blurring water highlights this. If a fast shutter speed is used, the movement would be frozen. This removes the dynamism of the water, as it appears static. Using a slower shutter speed to blur moving water restores the dynamism, creating visual interest. I frequently blur water for soft, delicate rendering. I also freeze it, when it is too bright for a slow shutter speed, or I want to freeze a pattern (like raindrops hitting a puddle).
Camera movement affects image sharpness. With a slower shutter speed, you are more at risk of camera shake. Faster shutter speeds generally prevent camera shake from softening images. As a rule, 1/focal length equals the lowest hand-holdable shutter speed. If I were to use a 28mm lens, the slowest I could hand-hold the camera without camera shake would be 1/28 second (or 1/30, the nearest faster shutter speed available on my cameras). Many cameras have image stabilisers, which reduce or eliminate the effects of camera shake, allowing slower handheld shutter speeds. Irrespective of this, a sturdy tripod and cable release remains the best means of obtaining sharp images. Certainly, they are the only way at long shutter speeds.
In summary, shutter speed controls how long light is entering the camera for. Measured from fractions of a second (short) up to hours (long), it controls how movement is perceived in a photograph. Camera shake can occur at longer shutter speeds, but the 1/focal length rule provides a good indicator of the slowest hand-holdable speed. Image stablisers allow more hand-holdability.
A simplistic way of viewing all this:
Short shutter speed = sharp subject
Long shutter speed = less sharp subject