Watch video: Understanding the exposure triangle
There are three main controls on your camera that must be coordinated in order to create a balanced exposure – an image with a good range of tones that is neither too bright nor too dark. These are shutter speed, aperture and ISO.
Shutter speed determines the amount of time that the sensor is exposed to light. The aperture is the size of the opening in the lens that this light passes through. And ISO is the sensitivity of the sensor to light. Change any one setting and you will also have to change one (or both) of the others to compensate.
While the scales used for these controls are all different – shutter speed is measured in seconds (or fractions of a second), aperture in f-stops, while ISO uses multiples of 100 – increasing or decreasing any of these settings to the next unit in its scale is achieved by doubling or halving its value, referred to as a ‘stop’.
So a shutter speed of 1/250sec can be doubled by one stop to 1/125 sec, or halved to 1/500sec. An aperture of f/5.6 lets in twice as much light as f/8, or half as much as f/4. And ISO400 is twice as sensitive to light as ISO200, and half as sensitive as ISO800.
Increasing any of these settings by a stop means that you will also need to decrease one of the other settings by a stop for the same overall exposure. However, in addition to determining the brightness of the image, these settings also have an important creative effect over the look of the image.
A fast shutter speed freezes movement, while a slow shutter speed will record anything moving as a blur. A narrow aperture gives greater depth of field than a wide aperture, meaning that more or less of the scene is in focus. And the greater the ISO, the noisier the image will appear. Let’s see how these settings can be used to our advantage…
To ensure that the model is isolated from the background, we used a wide aperture of f/2.8. Then, to compensate for the increased light coming through the lens, we balanced it with a very fast shutter speed of 1/3200 sec.
To blur the movement of the water, we need to set a long exposure time. Our shutter speed was set to 15 secs, so we stopped down our aperture to f/11 to restrict the light and increase the depth of field.
Shooting music gigs means working in low light, so we upped the ISO to 3200 in order to make the image sensor more sensitive to light. We used an aperture of f/1.4 to retain a fast shutter speed – enabling us to freeze the motion on stage.