What is exposure and how does it work?

Introduction to the exposure triangle

Exposure is the foundation of photography. It refers to the amount of light that reaches your camera sensor, determining how bright or dark your image will be.

More light = brighter image. Less light = darker image.

Too much light results in an image that is overexposed, which means the image becomes too white, losing detail in the highlights. Too little light results in an image that is underexposed, which means the image becomes too black, losing detail in the shadows.

Correct exposure sits between these two extremes, balancing three key settings: aperture, shutter speed and ISO, collectively known as the exposure triangle.

Aerial view of a mountainous landscape featuring a winding road and a small town with terracotta-roofed houses nestled into the hillside. The town is surrounded by green hills and stretches toward a coastal view in the distance.

Here is a correctly exposed image of a Sardinian landscape. Swipe left to see the over-and underexposed versions | Image © Pete Townshend

Aerial view of the same mountainous landscape as the previous image but with the exposure significantly increased, making the scene appear overly bright and washed out.

Here is the same image overexposed. This time, the highlights in the buildings have become blown out | Image © Pete Townshend

Aerial view of the same mountainous landscape as the first image but with the exposure drastically reduced, resulting in a much darker and underexposed scene.

Here is the same image underexposed. As you can see, the darker areas of the image have become almost black, losing all detail | Image © Pete Townshend

Not all cameras will allow you to control each element of exposure manually. For example, the camera on your smartphone will likely choose these settings automatically when you press the capture button, but that’s ok.

While it may give you more control, using manual exposure mode doesn’t make you a good photographer, just like using auto mode doesn’t make you a bad photographer. Indeed, we’ve met many professionals that use some form of automatic mode.

But possessing a strong understanding of how exposure works will make you a better photographer, regardless of how you use your camera. Knowing how your camera processes light will help you better understand how to use the light available in the scene to your advantage.

First, we’ll explore each of the three elements of the exposure triangle in turn (aperture, shutter speed and ISO), then we’ll look at how they interact.

What is aperture?

Aperture refers to the opening inside a lens that light passes through to enter the camera. In most cases the size of this opening is adjustable. Aperture size is measured in f-stops, which are generally preceded by a ‘f/’ (e.g., f/2.8, f/5.6, f/16).

Somewhat confusingly, the larger the f-stop number, the smaller the aperture and vice versa, but don’t worry, you’ll get used to it.

Large aperture (small f-stop number, e.g., f/2.8): More light enters, resulting in a brighter image.
Small aperture (large f-stop number, e.g., f/16): Less light enters, resulting in a darker image.

As well as exposure, aperture size also affects something called depth-of-field, which is the proportion of the image that is in sharp focus.

We’ll cover aperture vs depth-of-field in Lesson 2 of this course, so we’ll park that for now and just concentrate on exposure to keep things simple.

A horizontal aperture scale from f/1.4 to f/16, indicating how aperture affects image brightness. Larger apertures (smaller numbers) let in more light, making the image brighter, while smaller apertures (larger numbers) let in less light.

What is shutter speed?

Shutter speed refers to the duration the camera’s shutter remains open to let light hit the sensor. In other words, the amount of time the sensor is exposed to the light.

It is measured in seconds or fractions of a second (e.g., 1/1000sec, 1/250sec, 1sec).

Note that, when shutter speed is displayed on a camera, the ‘1/’ is often dropped from the fractions, so 1/1000sec or 1/250sec shutter speeds may appear as simply 1000 or 250 and so on.

Fast shutter speed or ‘short’ exposure (e.g., 1/1000sec): Less light enters, resulting a darker image.
Slow shutter speed or ‘long’ exposure (e.g., 1sec): More light enters, resulting in a brighter image.

Like aperture, shutter speed affects more than just the exposure of an image, it also has an effect on the way motion is captured.

We’ll go into more detail about shutter speed and how it affects motion in Lesson 3 once you have a good handle on basic exposure.

A horizontal shutter speed scale from 1/8 to 1/1000, showing how shutter speed affects brightness. Slower speeds make the image brighter, while faster speeds result in darker images.

What is ISO?

ISO is the camera sensor’s sensitivity to light, which can be adjusted, either manually or automatically depending on the camera. It is represented by a number, preceded by the letters ‘ISO’ (e.g., ISO 500, ISO 1000, etc). The higher the number, the higher the sensitivity.

ISO stands for International Organisation for Standardisation, which is the organisation that set the standard for digital camera sensors’ sensitivity to light.

The vast majority of modern digital cameras are capable of a 100-3200 ISO range, however, many offer sensitivities into the hundreds of thousands.

Here’s how it works:

Low ISO (e.g., ISO 100): Less sensitive to light, resulting in a darker image.
High ISO (e.g., ISO 3200): More sensitive to light, resulting in a brighter image.

As with the other elements in the exposure triangle, ISO affects more than just exposure. When sensitivity increases, something called ‘noise’ begins to appear in your images – a grainy effect that reduces detail. For this reason, it’s best to always use the lowest ISO possible in the conditions.

We’ll look more closely at ISO and noise management in Lesson 4.

A horizontal ISO scale from 100 to 12800, showing how ISO affects image brightness. Higher ISO numbers result in brighter images, while lower ISO numbers produce darker images.

The exposure triangle

The exposure triangle is a conceptual model showing the relationship between the three elements outlined above: aperture, shutter speed and ISO.

A specific amount of light is required to achieve correct exposure, so what you add to one element, you have to take away from one or both of the others to compensate.

Understanding this balance is crucial for achieving the desired exposure.

A diagram of the exposure triangle. The scales for aperture, shutter speed, and ISO are arranged in a triangle, showing how they balance brightness.

Put simply:

Increase the aperture to let in more light if you want to keep a low ISO and fast shutter speed.
Slow down the shutter speed if you need more light but want to keep ISO low and aperture small.
Increase ISO if the light is low but you want to maintain the aperture and shutter speed.

At this point you may be asking why you need to keep a fast shutter speed or a large aperture in the first place. Rest assured we’ll get into this in lessons 2, 3 and 4 – for now, here’s a practical exercise to help you get to grips with exposure.

Practical exercise

Understanding the exposure triangle

For this exercise, you’ll need a camera that lets you manually control aperture, shutter speed and ISO:

Take your camera outside in the daylight, set it to Auto mode and take a picture. This image will be your point of reference for correct exposure.
Now, set your camera to Manual mode (usually marked as ‘M’, if you’re unsure, look at your camera’s manual).
Once in Manual mode, adjust the aperture to f/8, the shutter speed to 1/125sec and the ISO to ISO 200.
Using those settings as a starting point, experiment with aperture: Take a photo at the largest aperture (lowest f-stop) and then at the smallest aperture (highest f-stop). Observe the differences in exposure.
Now, return the aperture to f/8 and experiment with shutter speed: Take a photo at a fast shutter speed (e.g., 1/1000sec) and a slower shutter speed (e.g., 1/60sec). Once again, observe the differences in exposure.
Finally, experiment with ISO: Return the shutter speed to 1/125sec, then take a series of photos, doubling the ISO each time (e.g., 200, 400, 800) and observe the differences.
Now you’ve experienced how each element affects exposure, it’s time to experience how they work together. Set the aperture to its largest setting and adjust the shutter speed and/or ISO to compensate until you match the exposure of the first photo. Alternatively, select a slower shutter speed such as 1/15sec and adjust the aperture to compensate, and so on. Most cameras will have an exposure scale on the viewfinder screen which you can use to help you achieve correct exposure.

Conclusion

Understanding exposure is fundamental to photography and having the ability to balance aperture, shutter speed and ISO gives you much more creative control over your images. Even simply understanding the concept will change the way you approach photography for the better.

Practice is key, so continue experimenting with different settings to see how they affect your photos.

We’d love to see how you’ve been putting what you’ve learned into practice. Share your images with us on social media and you could be featured!