THE EXPOSURE TRIANGLE

The exposure triangle is the term used to explain how ISO, aperture and shutter speed all interact to affect the exposure of an image. Aperture and shutter speed affect how much light the sensor is exposed to and ISO alters the sensitivity of the sensor to that light.

Shutter speed

The shutter speed on a camera is the length of time the shutter is open to allow light onto the sensor, it is usually measured in fractions of a second (such as 1/125, 1/60 etc). Although shutter speeds can be any length you want (for long exposure shots) anything longer than 1/60th (under normal light conditions) would cause your image to blur due to camera shake – so a tripod is always advisable. Faster shutter speeds let less light into the camera so can make your image darker, but in good light conditions fast shutter speeds can freeze action shots and give a good amount of detail – so can be used for sports photography or when your subject is moving (cars, trains, people). It is a trade off though and in order to use fast shutter speeds you do need good light. In good light conditions shutter speeds should be between 1/125” and 1/400th to allow a properly exposed image. In low light conditions when doing hand held photography you need to be careful not to slow the shutter down too far as you can end up with a blurry image – try adjusting ISO or aperture instead.

Aperture

The aperture element of the exposure triangle is quite simply “the size of the hole” that the light passes through – the bigger the hole, the more light enters the camera and the brighter your image will be. Aperture is measured in F-stops, the larger the F number the smaller the hole and each F-stop is half the size of the one before. If you want to “freeze” movement in lower light conditions you can try adjusting the aperture so you can still use a fast shutter speed to keep everything in focus.

ISO

The ISO on a DSLR is how sensitive the sensor is to light, a faster ISO makes the sensor react faster to light, meaning you can use faster shutter speeds in lower light conditions – the downside is you can end up with a lower quality image that is “grainy” or “noisy” so wherever possible always use the lowest ISO you can and adjust the aperture or shutter speed.

TTL Metering

TTL stands for “through the lens” and is how your DSLR helps you know you are ging to get a properly exposed image. If the meter is too far to the right (positive numbers) then your image will be over exposed and bright, if it's too far to the left it will be under exposed and too dark – positioning in the middle “0” will ensure you get a properly exposed image.