Exposure – allowing light to hit the camera’s sensor to record an image – is controlled by three variables: aperture, shutter speed and ISO. The size of the aperture determines how much light is let in through the lens, while the shutter speed dictates the duration of the sensor’s exposure to the light. ISO regulates the sensor’s sensitivity to light (technically it doesn’t – it controls how much the signal from the sensor is amplified).
You must understand the relationship between these variables, as each decision you make in terms of aperture, shutter speed and ISO will have an affect on the look and feel of a picture, as well as its brightness.
For example, aperture is a key ingredient for controlling the depth of field, or how much of an image appears sharp. Shutter speed also affects image sharpness, with slower shutter speeds leading to blurred images – whether that’s caused by the subject moving or the camera not being stationary during the exposure.
ISO enables you to use the best combo of aperture and shutter speed when the amount of light available to make an exposure would otherwise prevent it. But, upping the ISO risks reducing the quality of your shots.
This is where the exposure triangle comes in. The key is that when you increase the exposure for one variable (a green arrow), you need to decrease it for one or both of the other variables (the red arrows).
Exposure is essentially a juggling act between aperture, shutter speed and ISO. If you increase one of the three variables, then one or both of the other settings will need to decrease by an equivalent to maintain the same level of exposure.
Exposure can be measured in ‘stops’, with each stop representing double or half the level of exposure of the adjacent stop. If you increase the exposure by one stop, the sensor will receive twice the level of exposure. Decrease it by one stop, and the exposure is halved.
Aperture, shutter speed and ISO can each be described in stops. So, a shutter speed of 1/50 sec is one stop brighter than 1/100 sec, as the sensor is exposed for twice as long. But the same 1/50 sec speed is one stop darker than 1/25 sec. ISO is just as clear. Low numbers are less sensitive, higher numbers are more. A sensitivity of ISO400 being one stop brighter than ISO200.
The ranges of apertures on a lens are similar – opening up the aperture by one stop gives twice the level of exposure, while closing it by one reduces the exposure by half – but the sequence is less obvious. Larger f-numbers represent smaller apertures, while smaller f-numbers give larger apertures – just try thinking of them as fractions: a 1/16th is smaller than a 1/4.
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