EXP · Exposure
The Exposure Triangle: Aperture, Shutter Speed, and ISO
The exposure triangle is the single most important idea in photography. This guide explains how aperture, shutter speed and ISO trade off against each other in whole stops.
Diagrams
Every photograph is a balance of three controls: aperture, shutter speed and ISO. Together they form the exposure triangle, and each control is measured in stops, where one stop is a doubling or halving of the light reaching the sensor. Learn to think in stops and exposure stops being guesswork. Change one side of the triangle by one stop and you must change another side by one stop in the opposite direction to keep the same brightness.
What makes the triangle useful is that each control does something else as well as control light: aperture shapes depth of field, shutter speed decides how motion is recorded, and ISO decides how much noise you accept. That is the whole art of exposure — choosing which of those three side effects you want.
Thinking in Stops Instead of Numbers
A stop is a doubling or halving of light, and the three scales are built from it. Aperture runs f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16 in full stops. Shutter speed runs 1/500s, 1/250s, 1/125s, 1/60s, 1/30s, 1/15s and so on. ISO runs 100, 200, 400, 800, 1600.
Because every step is the same size in light terms, the trade is always one for one: open the aperture a stop and shorten the shutter a stop, and the brightness is unchanged. Most cameras can also move in thirds of a stop, which gives finer control without changing the logic.
Once you think this way, a scene becomes a set of choices rather than a list of numbers. The question is never what the correct exposure is, but which combination of the three gives you the sharpness, blur and noise you actually want.
Aperture: Light and Depth of Field Together
Aperture is the size of the hole in your lens, written as an f-number, and its scale runs backwards: f/2.8 is a wide opening and f/8 is a narrow one. Going from f/2.8 to f/4 halves the light; f/2.8 to f/8 is three stops, so eight times less light.
Aperture also sets depth of field, the zone that appears sharp. Use f/1.8 to f/2.8 for portraits with a soft background, f/4 to f/5.6 for couples and street scenes where some context should stay readable, and f/8 to f/11 for landscapes that need to be sharp from front to back.
Two caveats are worth remembering. Many lenses are slightly soft wide open and sharpen a stop or two down, and past f/16 diffraction begins to soften the whole frame. Most lenses are at their best somewhere between f/4 and f/11.
Shutter Speed: Light and Time Together
Shutter speed is how long the sensor is exposed, from about 1/4000s to 30 seconds on most cameras. 1/500s freezes a running child, while 1/60s is roughly the slowest speed you can hand-hold without blur on a standard lens. Every halving of time, from 1/125s to 1/250s to 1/500s, is one stop less light, so shutter speed and aperture trade directly against each other.
Slow speeds are not only a problem; they are a tool. Around 1/30s with a smooth pan blurs a cyclist's background while the subject stays recognisable, and 1 to 4 seconds on a tripod turns moving water into silk.
The rule of thumb for hand-holding is to keep the speed at or above one over the focal length — about 1/50s on a 50mm lens — and to raise the ISO when the light will not allow it. Image stabilisation buys you a few extra stops, but it does nothing for a subject that is moving.
ISO: Brightness Bought With Noise
ISO is the sensor's sensitivity setting. Base ISO, usually 100 or 200, gives the cleanest file; each doubling (200, 400, 800, 1600) adds one stop of brightness and progressively more noise. Modern cameras stay perfectly usable at ISO 3200 to 6400, but shadow noise and lost colour detail grow quickly beyond that.
It helps to know why noise appears at all. Noise is largely a consequence of a weak signal: the less light the sensor collects, the more the amplification has to work with, and the more visible the grain becomes. That is the practical argument for raising ISO only after you have exhausted aperture and shutter speed, and for choosing a faster lens when you frequently shoot indoors.
Two habits prevent trouble. Set a maximum ISO you are happy with so Auto ISO cannot silently push a file past it, and reset ISO to its base value when you finish shooting indoors, because a leftover ISO 3200 ruins the next daylight frame.
A Field Workflow in Three Steps
The order that works for most situations is: set the aperture for the depth of field you want, set the shutter speed for the motion you want, then raise ISO only as far as needed. Because aperture and shutter are chosen for visual reasons first, ISO absorbs whatever is left.
Applied to real scenes: for a running child, choose f/4 for a little separation, 1/500s or faster to freeze the legs, and let ISO fall where it must. For a landscape on a tripod, choose f/11, then 1/60s or longer, and keep ISO at 100. For a dim indoor party, open up to f/1.8, hold 1/60s to stop people moving, and accept ISO 3200.
Check the histogram rather than the rear screen, then adjust one control and only one. If the frame is too dark, open the aperture or lengthen the shutter; if the subject is blurred, do the opposite and raise the ISO instead. Auto ISO with a sensible minimum shutter speed is a good compromise when the light changes faster than you can react — it automates the last step of the workflow without touching the first two.
Key points
- One stop means a doubling or halving of light, so always balance both sides of the triangle in stops.
- Lower f-numbers let in more light: f/2.8 is three stops brighter than f/8.
- 1/500s freezes action, 1/60s needs a steady hand, and 1/15s needs a tripod.
- Raise ISO last, because it buys brightness at the cost of visible noise.
Frequently asked questions
- What exactly is one stop?
One stop is a doubling or halving of the light that reaches the sensor. It is the common unit that lets three different scales be compared: one full aperture step, one halving or doubling of shutter time, and one doubling of ISO are all one stop. Many cameras work in thirds of a stop, which is finer but follows the same arithmetic.
- Why should ISO be the last control I raise?
Because it is the only one of the three that buys brightness without giving you anything back visually — you lose cleanliness instead. Aperture and shutter speed always change how the picture looks, so they deserve to be chosen deliberately. Turn the ISO up once those two are set, and only as far as the light demands.
- Can I use Auto ISO in manual mode?
Yes, and it is a common way to work. You fix the aperture and shutter speed for creative reasons and let the camera float the ISO to keep the brightness steady, which is useful at events and for fast-changing light. Set a maximum ISO and a minimum shutter speed in the menu so the automation cannot make a decision you would not have made yourself.
- What settings should I start with for a dark indoor party?
Open the aperture as wide as your lens allows, commonly f/1.8 or f/2.8, hold the shutter near 1/60s so that people are not blurred by their own movement, and let ISO climb to whatever the room demands. If the pictures are still too dark, accept a slower speed and brace the camera, or use a flash rather than pushing ISO beyond the range you are happy with.
