Every photo’s brightness is set by three things: how wide the lens opening is (aperture), how long the sensor collects light (shutter speed) and how strongly the signal is amplified (ISO). Together they are called the exposure triangle. Change one, and you must change another to keep the same brightness. Each also has a side effect on how the image looks, and choosing those side effects is the creative part of exposure.
Thinking in stops
A stop is a doubling or halving of light. It is the common currency of the triangle: one stop more from the aperture can be paid for with one stop less from the shutter speed, and the photo is equally bright.
- Shutter speed: doubling the time is one stop. 1/500 → 1/250 → 1/125 → 1/60 s.
- ISO: doubling the number is one stop. 100 → 200 → 400 → 800.
- Aperture: the area of the opening doubles when the f-number is divided by about 1.4 (√2). The full-stop sequence is f/1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22 — smaller numbers mean more light.
Most cameras also move in thirds of a stop: f/4, 4.5, 5, 5.6 or 1/125, 1/160, 1/200, 1/250. Our exposure calculator rounds results to these real settings.
Aperture: brightness and depth of field
A wide aperture lets in a lot of light and gives shallow depth of field; a narrow aperture lets in less and keeps more of the scene sharp. Portrait photographers open up to f/1.4–f/2.8 for background blur; landscape photographers stop down to f/8–f/11 for front-to-back sharpness. Very small apertures (f/16 and beyond) soften the image through diffraction.
Shutter speed: brightness and motion
A fast shutter freezes motion; a slow one records it as blur. To freeze a running child or a bird in flight you need 1/500–1/2000 s; for silky waterfalls, half a second or more, often with an ND filter. Handheld, the old guideline is a shutter speed no slower than 1 ÷ (focal length × crop factor) — 1/50 s for a 50 mm lens on full frame — and image stabilisation buys several stops on top of that for static subjects.
ISO: brightness and noise
ISO does not change how much light reaches the sensor; it changes how brightly the camera renders it. Raising ISO lets you keep a fast shutter or a narrow aperture in dim light, at the cost of more visible noise because less light was actually collected. Modern full-frame cameras give clean results up to ISO 3200–6400; smaller sensors show noise earlier. A sharp, slightly noisy photo almost always beats a clean, blurred one.
Exposure value and equivalent exposures
Photographers summarise a combination of aperture and shutter speed with a single number, the exposure value: EV = log₂(N² ÷ t). Every combination with the same EV gives the same exposure at the same ISO. Normalised to ISO 100, EV also describes the brightness of the scene itself: full sun is about EV 15, bright overcast EV 13, a home interior at night around EV 5.
Here are equivalent exposures for a bright overcast scene at ISO 100:
| Aperture | Shutter speed | Look |
|---|---|---|
| f/1.4 | 1/4000 s | thin focus, frozen motion |
| f/2.8 | 1/1000 s | shallow focus |
| f/5.6 | 1/250 s | moderate depth |
| f/11 | 1/60 s | deep focus |
| f/22 | 1/15 s | deepest focus, diffraction, motion blur |
All of these photos would be equally bright, but they would look very different. That is the core idea: decide what matters most for the subject, set it, then balance the other two.
The sunny 16 rule
Before light meters, photographers used a simple rule: in bright sun with sharp shadows, expose at f/16 with a shutter speed of 1 ÷ ISO. At ISO 100 that is f/16 at 1/100 s (or the nearest, 1/125 s). From there you count stops: for a portrait at f/5.6, that’s three stops wider, so the shutter must be three stops faster — 1/800 s. In open shade or overcast light, open up two to three stops. The rule still works as a sanity check when your meter is fooled by snow, sand or a dark subject. The sunny 16 rule chart lists the shutter speed for every ISO and kind of daylight.
Which setting to prioritise
- Portraits: aperture first (f/1.8–f/2.8), shutter at least 1/250 s for people, ISO as needed.
- Sports and wildlife: shutter first (1/1000 s or faster), aperture wide open, Auto ISO.
- Landscapes: aperture first (f/8–f/11), lowest ISO, shutter as long as the tripod allows.
- Night sky: widest aperture, the longest shutter speed before stars trail (see the NPF rule calculator), then raise ISO to 1600–6400.
- Events indoors: fast lens wide open, 1/125–1/250 s, and accept ISO 3200–12,800.
Putting it into practice with camera modes
Aperture priority (A/Av) lets you set the aperture while the camera chooses shutter speed — ideal when depth of field matters. Shutter priority (S/Tv) is the reverse, for motion. Manual with Auto ISO is popular for action and events: you fix both aperture and shutter, and the camera adjusts ISO to keep the exposure right. Full manual is best for tripod work and long exposures, where light is constant and you want every frame identical. In every mode, exposure compensation (±EV) nudges the result brighter or darker, one third of a stop at a time.
A worked example
You meter a street scene at f/8, 1/125 s, ISO 100 (EV 13). A cyclist approaches and you need 1/1000 s to freeze them. That is three stops faster. You can open the aperture three stops to f/2.8, raise ISO three stops to 800, or split it: f/4 and ISO 400. The calculator does this arithmetic for any combination and shows the full table of equivalents.