Understanding and mastering the exposure triangle

The key to understanding photography lies in the exposure triangle, a concept that all photographers should seek to understand. The exposure triangle is the three camera settings that determine your exposure, that is, the brightness or darkness of your image. It includes: aperture, shutter speed, and ISO.

These three settings determine how much light your camera receives and how it responds. However, these settings influence more than just the brightness of your image. Let’s examine each component of the exposure triangle.

Aperture (f-stop)

The aperture is the opening in your camera lens that allows light in. It is measured in f-stops, such as f/1.8m, f/2.8, or f/8. Your camera will display these numbers when you change the size of the lens aperture.

The technical definition of f-stops is the ratio of the lens’ focal length to the diameter of the aperture opening. You don’t need to remember that definition. The important part is that lower f-stop numbers mean wider(larger) apertures, and higher f-stop numbers mean narrower(smaller) apertures.

When someone advises you to “shoot wide open,” they mean to photograph using the widest aperture your lens supports, which could be f/1.8, f/2.8, or f/4. When someone says to “stop down” your lens, photograph using a higher f-stop number(a smaller aperture).

How does the aperture affect photos?

• Wider apertures(e.g., f/1.8) allow more light to pass through the lens to the camera’s sensor, resulting in brighter images. Narrow apertures (e.g., f/16) allow less light through the lens and result in darker images.
• Wider apertures create a shallow depth of field, where your subject is in focus and your background is blurred. Wider apertures are often used in portrait photography for flattering effects.
• Narrow apertures increase the depth of field, resulting in images where more of the scene is in focus from front to back, which is often desired for landscape photography.

Shutter speed

Shutter speed is the camera setting that determines how long your camera’s sensor is exposed to light. It is measured in seconds or fractions of a second (e.g., 1/1000sec, 1/60sec, 2sec).

How does shutter speed affect photos?

• Slower shutter speeds allow more light to reach the camera’s sensor, leading to brighter images. A faster shutter speed allows less light, resulting in darker images.
• Shutter speed also determines how your camera captures and freezes or blurs motion in the scene. A fast shutter speed, like 1/1000 sec or 1/2500 sec, freezes the action of fast wildlife or sports. Slower shutter speeds, like 1/30 sec, 1 sec, or longer, allow motion blur in your scene. They are often used for smoothing water or panning motion blur effects.

ISO

ISO is a setting derived from film, referring to a film’s sensitivity to light. For example, ISO 400 film is four times more light-sensitive than ISO 100 film, resulting in brighter images.

ISO serves a similar purpose in digital cameras, though the sensitivity of camera sensors doesn’t change. Think of ISO as a “volume dial” for brightness.

Your camera sensor emits a signal based on the amount of light allowed in based on shutter speed and aperture. Increasing the ISO amplifies the signal for darker images, resulting in a brighter picture.

Digital cameras have a base (or native) ISO, the lowest setting providing the least digital noise (e.g., ISO 100, ISO 64).

How does ISO affect photos?

• Higher ISO increases brightness for low-light environments. Lower ISO is best for bright scenes.
• Increasing ISO also increases digital noise, making the image grainy. Using the lowest possible ISO results in the least digital noise.

How the exposure triangle interacts

Controlling the exposure triangle is key to obtaining the best photographic results. These three settings interact and must be balanced based on your photographic situation. Here are a few examples.

• You are photographing fast-moving wildlife during golden hour because the color of the light is beautiful. To freeze the action of the wildlife, you fix your shutter speed at 1/1000 sec, letting in less light. During golden hour, the quantity of light is lower, so you open up your aperture to the maximum that the lens supports. However, the image is still too dark, so the ISO must be increased to obtain the correct brightness for your image.
• You are photographing a group of animals that are staggered in depth. You set your shutter speed to 1/500 sec to freeze their motion and then chose a small aperture of f/16 to get the entire group in focus. Because of the faster shutter speed and small aperture, less light reaches your sensor, and a higher ISO is needed.
• You are photographing people indoors without a flash. The quantity of light is relatively low. You fix your shutter speed at 1/250 sec to freeze people’s motion. You choose a wide-open aperture to let in the most light, but you must increase the ISO to obtain the correct brightness.
• You are creating a portrait of someone using a very shallow depth of field. To freeze their motion, you set your aperture to wide open at f/1.8 and your shutter speed to 1/320 sec. Because you are letting in a lot of light to the sensor, you need to raise the shutter speed or lower the ISO to prevent overexposure.
• You are photographing the stars on a dark, moonless night. You aim to gather as much light as possible to the sensor, so you set the aperture to its maximum at f/2.8. The longest shutter speed you can use is 10 seconds to create pinpoint stars, not star trails. Using those two settings, you set the ISO to a higher setting to achieve the correct brightness.

Camera modes

Digital cameras have four basic photo modes, each determining which exposure settings are controlled by the photographer and which are decided by the camera. The camera makes these setting decisions using a built-in light meter. Each mode offers the convenience of having the camera make decisions for you. However, each mode has its drawbacks.

• Program (P) or Auto: The camera decides shutter speed, aperture, and ISO.
  • Program mode is the easiest mode to operate, but the camera might not make the correct decision for the scene in front of you. For example, if you photograph fast-moving birds, the camera might choose a shutter speed that is too slow to freeze their motion. Or if you are trying to photograph a group of people at a staggered depth, keeping them all in focus, the camera might choose an aperture that is too wide to achieve your depth of field.
• Shutter priority (S or Tv): The photographer decides the shutter speed, and the camera chooses the aperture and ISO.
  • Shutter priority mode is convenient when the shutter speed is crucial to freezing motion. You fix the shutter speed, but the camera makes decisions about aperture. However, when depth of field and freezing motion are essential, shutter priority mode doesn’t always work.
• Aperture priority (A or Av): The photographer decides the aperture, and the camera chooses the shutter speed and ISO.
  • Aperture priority is convenient when your primary goal is to achieve a certain depth of field. Most cameras let you set a minimum shutter speed in Aperture priority mode to freeze motion, but what if that is not enough? For example, a warbler perched on a branch can be photographed sharply with a minimum shutter speed of 1/500 sec or less. The moment the warbler takes flight, you need a shutter speed of 1/2000 sec or faster to freeze the motion of its wings.
• Manual (M): The photographer decides the aperture, shutter speed, and ISO. Enabling auto-ISO in this mode allows the camera to set the ISO while the photographer maintains control of aperture and shutter speed.
  • Manual mode offers the photographer the most control, but is the most complex. Once you are comfortable with how the exposure settings interact, it lets you make creative and intentional decisions about shutter speed, aperture, and ISO.

Our recommendation

Taking control of these settings can be intimidating or confusing when learning photography. It takes practice and experience to understand how aperture, shutter speed, and ISO affect your final photo. Use modes like Shutter priority and Aperture priority to learn how each affects your photographic results.

Once you are comfortable with each setting, you’ll eventually graduate to using Manual mode and will appreciate the ability to make intentional and creative choices when capturing photos.