The exposure triangle
There are three main elements that determines how a photograph is going to be exposed to light, and how that light generates a signal that can be recorded. Together, these three elements are known as “the exposure triangle”. They are:
- ISO – the photographic unit of measure for light sensitivity. The ISO you set determines the sensitivity the digital camera's sensor to light. However, increased sensitivity comes at a cost because increasing the ISO also increases noise. The higher the ISO, the higher the noise.
- Aperture – the size of the opening in the lens that let the light in to expose the sensor to light. The aperture determines the depth of field in a photograph. The smaller the aperture, the larger the depth of field.
- Shutter speed – the time that the shutter is open and let the digital sensor be exposed to light. The shutter speed determines the motion blur in a photograph. The longer the shutter speed, the more motion blur to we get from moving objects and camera movements.
The three elements, and their what aspect they each determine, is shown in the drawing below.
Understanding the relationship between these three elements is the key to understanding exposure.
For maximum sharpness, we want no noise in our photographs, maximum depth of field and no motion blur. Why don't we just set a low ISO, a small aperture, and a high shutter speed? Unfortunately, such a combination may not give the image the right exposure. It will probably be too dark. The amount of light available when we take a photograph is what determines the exposure. What we can do, is to select values for elements at the three corners of the exposure triangle that changes the priorities between what the elements determine.
In other words, to have low noise and a large depth of field, we can select a slow shutter speed in order to use a low ISO and a small aperture. A slow shutter speed, we know, may give us motion blur, so we can only use such a setting with stationary subjects and the camera on tripod. If we want a short shutter speed in this situation, we can instead increase the ISO. This makes the motion blur go away, but we will have noise in our photographs.
When working with the triangle, you need to know that change in one of the elements will usually make it necessary to change one or both of the others. This means that you can never really isolate just one of the elements alone. You always need to change at least two elements, and understand what impact a change in one is going to have on the two others.
ISO in digital photography is a linear scale where a number such as “200” or “400” is a the measure of a digital camera sensor's sensitivity to light.
The scale is designed so that doubling the number indicates a doubling in sensitivity. In other words, setting the ISO to a digital sensor to “400” makes it twice as sensitive to light as setting it to “200”.
For more details about the ISO scale, see our article: ISO in Digital Photography.
The aperture setting is a number that is a measure of the size of the opening in the lens, for example “f/2.8”.
The aperture determines the depth of field. A large aperture gives a photograph a shallow depth of field, and a small aperture gives a large depth of field.
For more details about the aperture, see our article: The Aperture and the F-number.
The shutter speed is the time that the shutter is open and let the digital sensor be exposed to light. Very long shutter speeds is expressed in whole numbers such as “1 second”, “2 seconds”, and so on. But the shutter speeds we most often use are expressed as fractions, such as “1/125 second”, which is the same as “0.008 second”.
Doubling the shutter speed, for instance from “1/125 second” to “1/250 second”, lets only in half as much light.
Using the exposure triangle
Now that we know how all the three elements that makes up the exposure triangle operates, we can also see how they work together.
For instance, on a clear and sunny day, outside at noon, the correct exposure is probably ISO 100, f/16, and 1/100 second. (This particular combination of the three elements of exposure triangle is known to photographers as “sunny sixteen”.)
Let say we want a faster shutter speed – 1/200 second – but the same exposure. We can do this by either doubling the ISO or by opening up one stop. In other words, we will get the same exposure by setting ISO 200, f/16, and 1/200 second, or by setting ISO 100, f/11, and 1/100 second.
Understanding the exposure triangle makes you able to take creative control of your photography. First: It will help you to know what to settings to use for achieving special effects such as motion blur or shallow depth of field. Second: It will also stop you from having unwanted effects in your photos, such as excessive noise from at too high ISO-setting, or motion blur when you actually wanted to use the camera to freeze motion.