When you shoot a photograph, what you’re really doing is taking the light you see and capturing it on film or in a digital file. In fact, the word ‘Photography’ is a combination of two Greek words which literally mean “light drawing”. To create a good “light drawing” it’s important to understand how cameras control the amount of light captured. This is called exposure, and it refers to the amount of light passing through the camera lens and reaching the camera sensor.
The Exposure Triangle
Exposure can be best understood using what is often referred to as the exposure triangle. The exposure triangle is the way a camera maintains the correct exposure for an image, and is composed of three elements: aperture, shutter speed, and ISO. In the exposure triangle, these three elements are related to each other, and if one of them changes at least one other must change to maintain the correct exposure.
Aperture refers to the diameter of the hole in the lens that lets the light in. The larger the diameter of the hole, the more light can reach the film or the camera sensor.
Aperture diameter is measured in f-stops. An F-stop refers to the ratio of the focal length of the lens to the diameter of the aperture. The important thing to remember is that the smaller the f-stop number, the larger the diameter of the aperture.
Mechanically, the aperture is part of the lens, not the camera body. When you hear someone speak of a “fast lens” they are talking about a lens with a large maximum aperture, such as f/1.8.
Shutter speed refers to the length of time the camera’s shutter is held open, allowing light to hit the film or camera sensor. Shutter speed is measured in fractions of a second. You will often see shutter speeds written as 1/250 or 1/125. For a shutter speed of 1/250, the camera’s shutter is held open for one two-hundred-and-fiftieth of a second.
In low light settings, longer shutter speeds are often used to get a correct exposure. On most modern cameras, the longest automatic shutter speed available is typically 30 seconds.
For beginning photographers, ISO can be the most confusing part of the exposure triangle, or at least it was for me when I started out. In film photography, ISO is also referred to as film speed. This refers to how sensitive the film is to light.
In digital photography, ISO can be thought of as how sensitive the camera sensor is to light. This is not technically correct, as ISO is really an algorithm run by the camera software, but it helps to think of it this way.
With ISO, the larger the number, the more sensitive the film or camera sensor is to light. Higher ISO numbers allow the camera sensor to capture more light, but higher ISO numbers also introduce more noise into an image. For this reason, it’s usually recommended to shoot using the lowest ISO number you can get away with.
Choosing the Correct Exposure
Unless you are using Manual mode, your camera will adjust at least one of these settings for you. Cameras have a lot of different modes which are beyond the scope of this article, but a good setting for beginners is “aperture priority” mode. This is usually shown as an A or Av on your camera’s dial.
Aperture priority mode allows a user to set the aperture and ISO values, and the camera will choose the correct shutter speed to maintain a proper exposure. If you find the shutter speed is too slow, you will need to adjust either the aperture or ISO to compensate.
I’ve mentioned correct exposure several times in this article, but it’s also important to note that proper exposure is as much a choice of the photographer as it is a technical aspect of photography. Sometimes photographers, including myself, will intentionally over- or under-expose an image during capture. The original exposure can then be adjusted later in post-processing or maintained as an artistic choice. Whatever exposure choices you make, remember to always have fun and don’t be afraid to experiment.