Features

Photography is all about exposure - that is, allowing the right amount of light into the camera to generate a perfect image. Photographic exposures consist of just two variables: the amount of light the lens allows into the camera and the amount of time the sensor is exposed to it.

The period of time for which a sensor is exposed is known as the exposure itself, and it is measured by a shutter speed. The term 'shutter speed' refers to a physical curtain that opens and closes within the camera body. While digital SLRs still employ them, many digital compact cameras just emulate the effect by switching their sensor on and off for the required period.

Typical shutter speeds for daytime photography are generally a small fraction of a second. An exposure of 1⁄100 or 100th of a second is typical, although faster shutter speeds are better at freezing action, while slower ones can blur it. Photos taken at night require exposures that could last several seconds.

An adjustable iris called the aperture controls the amount of light a lens allows into a camera. If the aperture is wide open, it will allow in more light than when it's closed down to a small diameter. The diameter of the aperture can also control how much of the image is in sharp focus.

So the technical side of exposure simply involves finding the right combination of aperture and shutter speed for the camera's sensitivity and the current lighting conditions. The amount of light required for an ideal exposure is programmed into modern cameras. The camera simply uses its light meter to measure the brightness of the subject, then looks up the ideal exposure settings to record it.

Since most photos consist of light and dark areas, though, the question is which area should be measured. Early cameras used spot meters, which measured only the brightness in the very middle of the frame. Later models offered enhanced metering known as centre-weighted metering, which measured the brightness in a larger area, but still biased to the middle. With both systems you'd typically point the camera first at the area you wanted exposed correctly for a reading before recomposing the shot.

Today, more sophisticated metering systems split the frame into multiple segments and measure the light in each. Then they attempt to figure out the kind of photo you're trying to take and ignore areas that might influence the metering incorrectly. Some cameras do this better than others, but the best employ hundreds of metering segments and a library of 'perfect' images to refer to.

Most cameras offer the choice of spot, centre-weighted and segmented metering systems, and for general purposes the latter is the best. If you find your camera is getting a certain exposure wrong, you could try a different metering system to judge better the area you want exposed correctly.

	1 Basic metering systems can be fooled into underexposing by bright areas in a composition.

	2 Superior metering systems do a better job of judging the entire scene.