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# The exposure triangle, in plain words
- URL: https://syllabus.ghost-themes.apps.codememory.com/exposure-triangle/
- Published: 2026-08-25T09:00:00.000Z
- Updated: 2026-08-25T09:00:00.000Z
- Description: Aperture, shutter speed and ISO: what each one does, what it costs and how to balance them.
- Author: Ada Okafor
- Tags: Photography Essentials, #chapter Exposure, #Import 2026-09-30 07:57

Every photograph is a measured amount of light. Too little and the picture is dark; too much and it is washed out. Your camera controls that amount with three settings: aperture, shutter speed and ISO. Together they are called the exposure triangle. This lesson explains each one in plain words, and shows how changing one means changing another.

![The aperture is the opening inside the lens.](https://syllabus.ghost-themes.apps.codememory.com/content/images/2026/09/exp-lens-1.jpg)

The aperture is the opening inside the lens.

## A bucket in the rain

Imagine you want to fill a bucket with rainwater. You can use a wider bucket (more water arrives at once), leave it out longer, or be happy with a bucket that is less full. Photography works the same way:

- **Aperture** is how wide the bucket is: the size of the opening in the lens.
- **Shutter speed** is how long you leave the bucket out: how long the sensor sees the light.
- **ISO** is how sensitive the sensor is: how full the bucket needs to be for a good picture.

Each setting is measured in **stops**. One stop more means twice as much light; one stop less means half as much.

## Aperture

Aperture is written as an f-number: f/1.8, f/4, f/8, f/16\. The confusing part is that a **small number means a big opening**. f/1.8 lets in a lot of light; f/16 lets in very little.

Aperture also controls **depth of field**: how much of the picture is sharp from front to back. A wide aperture (f/1.8) gives a shallow depth of field, with a sharp subject and a soft, blurred background. A narrow aperture (f/11) keeps almost everything sharp.

![f/2: the face is sharp, the background melts away.](https://syllabus.ghost-themes.apps.codememory.com/content/images/2026/09/exp-portrait-1.jpg)

f/2: the face is sharp, the background melts away.

## Shutter speed

Shutter speed is how long the shutter stays open, written in seconds or fractions: 1/1000, 1/125, 1/2, 2". Fast speeds freeze movement; slow speeds blur it.

| Shutter speed    | Good for                               |
| ---------------- | -------------------------------------- |
| 1/1000 s         | Sport, birds, splashing water          |
| 1/250 s          | People walking, children playing       |
| 1/60 s           | Still subjects, handheld               |
| 1/4 s and slower | Tripod only: light trails, silky water |

A simple rule for handheld photos: use a shutter speed at least as fast as **1 divided by your focal length**. With a 50 mm lens, use 1/50 s or faster to avoid camera shake.

![Four seconds on a tripod turns car lights into lines.](https://syllabus.ghost-themes.apps.codememory.com/content/images/2026/09/exp-night-1.jpg)

Four seconds on a tripod turns car lights into lines.

## ISO

ISO sets how sensitive the sensor is. ISO 100 is the cleanest. Each doubling (200, 400, 800) makes the picture one stop brighter but adds more **noise**: grain and blotchy colour, especially in shadows. Modern cameras and phones handle ISO 1600 or 3200 well, so do not be afraid to raise it in dim light. A sharp, slightly noisy photo is better than a blurred, clean one.

## Balancing the triangle

Change one setting and, to keep the same brightness, you must change another by the same number of stops. For example, if you open the aperture from f/8 to f/5.6 (one stop more light), you can make the shutter speed one stop faster, from 1/125 to 1/250\. The brightness stays the same, but the look changes: less depth of field and more frozen movement.

💡

Start in aperture-priority mode (A or Av). You choose the aperture; the camera chooses the shutter speed. Keep an eye on the speed and raise the ISO if it gets too slow.

#### What about phones?

Phones have a fixed aperture, so they balance shutter speed and ISO for you. The same ideas still explain why night photos look grainy or blurred.

## Camera modes

Most cameras offer a few main modes:

- **Auto** chooses everything. Good for quick snapshots, but you cannot control the look.
- **Aperture priority (A or Av)** lets you choose the aperture; the camera sets the shutter speed. Best for most people, most of the time.
- **Shutter priority (S or Tv)** lets you choose the shutter speed; the camera sets the aperture. Useful for sport and movement.
- **Manual (M)** means you choose everything. It is useful when the light is steady, such as in a studio or for night photos on a tripod.

Many photographers use aperture priority with **auto ISO** and a minimum shutter speed. This gives creative control with very few missed shots.

## Practice: one scene, three settings

Choose a subject with a background a few metres behind it, such as a plant in front of a fence. Take three photos:

1. At the widest aperture your lens allows, such as f/1.8 or f/4.
2. At f/8.
3. At f/16.

Keep the camera in aperture priority so the brightness stays the same. Compare the backgrounds. Then repeat the exercise with shutter speed and a moving subject, like a fountain or traffic. Seeing the difference with your own eyes makes the triangle stick.

![](https://syllabus.ghost-themes.apps.codememory.com/content/images/2026/09/exp-g1-1.jpg)

![](https://syllabus.ghost-themes.apps.codememory.com/content/images/2026/09/exp-g2-1.jpg)

![](https://syllabus.ghost-themes.apps.codememory.com/content/images/2026/09/exp-g3-1.jpg)

Depth of field at work: from a soft background to a scene that is sharp all the way through.

📌

Key takeaways: aperture controls depth of field, shutter speed controls movement, ISO controls noise, and each one step is a stop.

## Quick check

Which aperture lets in the most light?

- f/16
- f/8
- **f/1.8**

You want to freeze a cyclist. Which shutter speed?

- 1/4 s
- 1/30 s
- **1/1000 s**

What is the cost of a high ISO?

- A shallower depth of field
- **More noise in the picture**
- More motion blur