Understanding Exposure Part 3

Controlling Exposure: Shutter Speeds & Apertures

Photographic exposure is a product of two factors; Shutter Speeds and Lens Apertures.

Shutter Speeds

Cameras have a shutter which opens and closes to regulate the amount of time light strikes the sensor (or film). A slow shutter speed is one where the shutter remains open for example 1/15th of a second. A fast shutter speed such as 1/1000s allows much less light to reach the sensor.

A shutter speed of 1/500s allows twice as much light to hit the sensor as a 1/1000s. 1/250s allows twice as much as 1/500s. 1/125s lets in double the amount of light as 1/250s and so on. Each doubling or halving of the exposure time is described as 1 "stop".

Low light levels require the shutter remain open longer to let enough light reach the sensor and form an image.

Shutter speeds can be set in either Shutter Speed Priority mode ("Tv" on Canon or "S" on Nikon) or in Manual Exposure mode.

Shutter speeds are logical and easy to understand; a brief exposure time (fast shutter speed) gives less exposure than a long exposure (slow shutter speed).

The table below shows the typical range of shutter speeds on modern DSLRs.

Apertures

The other factor controlling exposure is the lens aperture.

Inside the lens there is a diaphragm that opens and closes to let more or less light pass through the lens to reach the camera's sensor. Obviously a bigger opening or aperture will let in more light than a small aperture.

The size of the aperture is described in terms of "f-stops" e.g. f/4 or f/22. New photographers are often confused by the fact that f/4 is a relatively large aperture and f/22 is a very small aperture.

The f-stop equals the focal length of lens divided by the diameter of lens opening. E.g. a 100mm lens with a lens opening of 25mm diameter as an aperture of f/4 (100 divided by 25)

The table below shows the standard f-stops. f/8 lets in twice as much light as f/11 and lets in half as much light as f/5.6

Shutter speed and Aperture together determine Exposure

Any particular exposure value can be produced by a range of shutter speed/aperture combinations. For example f/8 @ 1/250s will give exactly the same exposure as f/11 @ 1/125s or f/5.6 @ 1/500s.

If you reduce the aperture one stop from f/8 to f/11 you can keep the same exposure by increasing the exposure time one stop from 1/250s to 1/125s.

A reasonable question might be: If various combinations of shutter speeds and f-stops give identical exposure values why worry which combination is used?

The answer is that while speeds and apertures together determine exposure values, each component controls a second important aspect of photography. Shutter speeds control the way motion is depicted in your shot. Apertures control "Depth of Field".