Chapter 1Part 1: Exposure
The ultimate guide to perfect photographic balance. Change aperture, shutter speed and ISO in the playground, then take the same decisions to your camera.
Think of the camera sensor as a canvas and light as the paint. Aperture and shutter speed determine how much light reaches it; ISO changes how brightly the captured signal is rendered. Together, these settings form the exposure triangle.
They also shape the photograph beyond brightness. You can keep a mountain landscape sharp, freeze a moving athlete, or soften a distracting background. Learning how the settings trade off gives you control over those choices.
Simplified. Real background blur also depends on the lens focal length and how far away the subject is. Real motion blur depends on how fast the subject moves. Noise also depends on how much light the sensor collects, not just the ISO setting.
Baseline exposure: f/4, 1/250 s, ISO 100.
A stop is the common unit for changing exposure. Add one stop with aperture or shutter speed and the sensor receives twice as much light; subtract one and it receives half as much. Most cameras let you adjust in one-third-stop clicks, so three clicks make one full stop.
+1 stopTwice the captured light
−1 stopHalf the captured light
The one-third-stop section works through the smaller steps and their maths. ISO is different: raising it brightens the recorded result without adding light at the sensor. The ISO chapter explores that trade in detail.
The table shows one-stop changes from a common starting point. The aperture and shutter columns change captured light; the ISO row changes recorded brightness.
| One stop brighter | Baseline | One stop darker | |
|---|---|---|---|
| Aperture | f/2.8f/3.2, f/3.5 between | f/4 | f/5.6 |
| Shutter | 1/1251/160, 1/200 between | 1/250 | 1/500 |
| ISO | 200125, 160 between | 100 | 50only on some cameras |
Each setting changes brightness, but each also has a creative effect. Choose the look you need first, then balance the remaining settings.
Aperture is the opening inside the lens. A small f-number such as f/1.4 means a wide opening and more captured light; f/16 means a narrow opening and less light.
It also affects depth of field. Around f/1.4–f/2.8 you can soften the background behind a portrait. Around f/8–f/16 you can keep more of a landscape sharp, depending on focal length, focus distance and subject distance.
Shutter speed is how long the sensor gathers light. A brief 1/1000 s or 1/2000 s exposure captures less light; 1/2 s or 30 s captures much more. Cameras may time this with a mechanical or electronic shutter.
Short exposures can freeze an athlete or bird. Long exposures can turn waves into mist or traffic into light trails. The result also depends on how fast the subject and camera move.
ISO controls the brightness of the recorded result, not the number of photons captured. The familiar sequence is 100, 200, 400, 800, 1600 and 3200: each doubling raises the result by about one stop.
Low ISO often gives the most room for clean shadows and highlights. Higher ISO helps when you need a faster shutter or smaller aperture in dim light; noise can become more visible because those choices collect less light. The amount depends on the camera and the scene.
To keep the same picture brightness, a change in one setting can be offset by an equal change elsewhere. This is exposure reciprocity. The visual result can still change: a sharper moving subject, softer background or more visible noise.
Suppose f/8, 1/125 s and ISO 100 give you the brightness you want, but the car is blurry. Moving the shutter from 1/125 → 1/250 → 1/500 s makes it two stops faster. The sensor now collects one quarter as much light, so the image will be two stops darker unless you compensate.
The playground starts from a different balanced example: f/4, 1/250 s, ISO 100. Use its controls to make the same two-stop trade.
The standard full-stop aperture sequence is f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22. Why isn't it simply 1, 2, 3, 4? Each circle below is drawn to scale for the same lens, and each one lets in half the light of the one before it.
The f-number is the lens focal length divided by the diameter of the opening. On a 50 mm lens, f/2 is a 25 mm opening and f/4 is 12.5 mm. That is why a bigger number means a smaller opening.
The amount of light depends on the area of the circular aperture:
A = πr2
To halve the area, the diameter doesn't need to halve. It only needs to shrink by a factor of
√2 ≈ 1.414
That's where the f-stop sequence comes from.
Take f/2 and multiply by 1.414:
2 × 1.414 = 2.828 ≈ 2.8
So the next full stop is f/2.8. Then:
2.8 × 1.414 ≈ 4
Keep going and you get f/2, f/2.8, f/4, f/5.6, f/8. Each step gives half as much light.
A smaller f-number means a larger physical aperture, and more light.
Modern cameras usually adjust aperture, shutter speed and ISO in 1/3-stop increments, not only full stops. Each click is 1/3 stop, so three clicks make one full stop. Your camera may show:
3 clicks, 1 stop less light
3 clicks, 1 stop less light
3 clicks, 1 stop brighter
A full stop doubles or halves the light. A third of a stop changes it by
21/3 ≈ 1.26
So each 1/3-stop click is roughly 1.26× the light, rather than 2×. Three clicks give 1.26 × 1.26 × 1.26 ≈ 2.
Light goes with the diameter squared, so the f-number only needs to change by the square root of one click of light, √(21/3):
fnext = fcurrent × 21/6
That multiplier is about 1.122. Starting from f/2.8:
Why is 3.14 marked f/3.2 and not f/3.1? The true value of f/2.8 is 2.83 (2 × √2), and 2.83 × 1.122 ≈ 3.17. The same happens with f/3.5, whose true value is 3.56, so the next click lands exactly on 4.
The numbers on screen are rounded labels, and the camera uses the exact values. For example f/5.6 is really 5.66, f/11 is 11.3, and 1/125 s is really 1/128 s. The practical sequence is:
Bold values are full stops.