Lesson 3: The Exposure Triangle: An Overview

Every photograph is an exposure: a measured amount of light allowed to reach the sensor. The exposure triangle is the model photographers use to think about it. Its three corners are aperture, shutter speed and ISO. Together they decide how bright or dark the photograph is, and each one also changes the look of the image in its own way. Master the triangle and you can take control of any camera.

The unit that ties the three corners together is the stop. One stop is a doubling or halving of the amount of light. Open the aperture by one stop and twice as much light reaches the sensor; shorten the shutter speed by one stop and half as much light gets in; raise the ISO by one stop and the sensor becomes twice as sensitive. Because all three are measured in the same stops, they trade against each other exactly: one stop less light from the aperture can be balanced by one stop more from the shutter or the ISO.

The Three Corners

Aperture is the size of the lens opening, measured in f-stops. A wide aperture (a small f-number such as f/2) lets in a lot of light and gives a shallow depth of field, with the background blurred. A narrow aperture (a large f-number such as f/16) lets in little light and keeps much more of the scene sharp. Shutter speed is how long the shutter stays open: a fast speed such as 1/1000 of a second freezes motion, while a slow speed such as 1/4 of a second lets moving things blur. ISO is the sensor's sensitivity to light: ISO 100 is least sensitive and cleanest, while ISO 3200 lets you shoot in dim light at the cost of visible grain, called noise.

The classic full-stop sequences are worth memorising. Aperture: f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22 — each step halves or doubles the light. Shutter speed: 1/1000, 1/500, 1/250, 1/125, 1/60, 1/30, 1/15, 1/8, 1/4, 1/2, 1 second — each step doubles or halves the time. ISO: 100, 200, 400, 800, 1600, 3200 — each step doubles the sensitivity. Modern cameras also offer third-stop steps between these, but the full stops are the skeleton.

Trading Stops in Practice

Suppose you are photographing a portrait outdoors at f/2, 1/500 second and ISO 100, and the exposure is correct. You decide you want more of the background in focus, so you narrow the aperture to f/4 — two stops less light. To keep the same brightness you must add two stops back: slow the shutter to 1/125 second (two stops slower), or raise the ISO to 400, or one stop of each. The photograph will be equally bright, but it will look different: more depth of field, or more motion blur, or more noise. That is the creative heart of the triangle — exposure is a technical problem with many solutions, and you choose the solution whose side effects you want.

When you are starting out, use the camera's semi-automatic modes to practise one corner at a time: aperture priority lets you set the aperture while the camera picks the shutter speed, and shutter priority does the reverse. Watch what the camera chooses and ask why. Within a few weeks you will find yourself predicting the trade-offs before you press the shutter — and that is the moment the triangle stops being a diagram and becomes a habit.