The exposure triangle explained: aperture, shutter speed and ISO for beginners
TUREFF• •0
Every photograph is a measured amount of light. The exposure triangle is the name for the three settings that control how much light reaches the sensor and how bright the final image looks: aperture, shutter speed and ISO. Each one also changes something else about the picture, and understanding those side effects is what moves you from auto mode to deliberate photography. Everything here applies to phones with a manual or pro mode as well as cameras.
Stops: the common currency
Photographers measure light in stops. One stop more means double the light; one stop less means half. All three settings can be adjusted in stops, which is why they trade against each other: open up one stop in one setting and close down one stop in another, and the brightness stays the same.
Aperture: how wide the lens opens
Aperture is the opening in the lens, written as an f-number: f/1.8, f/2.8, f/4, f/5.6, f/8, f/11, f/16. The numbers feel backwards: a smaller f-number means a wider opening and more light.
The side effect is depth of field, how much of the scene from front to back looks sharp.
- Wide aperture (f/1.8 to f/2.8): shallow depth of field. A sharp subject with a soft, blurred background, which suits portraits.
- Narrow aperture (f/8 to f/16): deep depth of field. More of the scene in focus, which suits landscapes and groups.
Most phones have a fixed aperture and simulate background blur with software in portrait mode.
Shutter speed: how long the light is let in
Shutter speed is how long the sensor is exposed, written in seconds or fractions: 1/1000, 1/250, 1/60, 1/2, 1s.
The side effect is motion.
- Fast speeds (1/500 and quicker) freeze movement: sport, children, birds, splashing water.
- Slow speeds (1/30 and slower) blur movement: silky waterfalls, light trails from traffic, a sense of speed.
- Very slow speeds also blur from your own hand shake, so you need a tripod or something steady to rest on.
A common rule of thumb for hand-holding is a shutter speed of at least one over the focal length (1/50 at 50mm), though image stabilisation lets you go slower.
ISO: how much the signal is amplified
ISO sets how strongly the camera brightens the signal from the sensor. Low ISO (100 or 200) gives the cleanest image. Raising it (1600, 3200, 6400) lets you shoot in dim light without a slow shutter or a wider aperture.
The side effect is noise: grain, speckles and smeared detail, worse in the shadows. Modern sensors handle high ISO far better than older ones, and a sharp, slightly noisy photo usually beats a blurred clean one.
How the trade-offs work in practice
Decide what matters most for the shot, set that first, then balance the others.
- Portrait at dusk: wide aperture for background blur, shutter fast enough to avoid shake, ISO up as needed.
- Running dog: fast shutter to freeze the action, then open the aperture and raise ISO to compensate.
- Landscape on a tripod: narrow aperture for front-to-back sharpness, low ISO for clean detail, and let the shutter be as slow as it needs to be.
Semi-automatic modes
- Aperture priority (A or Av): you choose the aperture; the camera picks the shutter speed.
- Shutter priority (S or Tv): you choose the shutter; the camera picks the aperture.
- Manual (M) with auto ISO is a popular middle ground: you set aperture and shutter, the camera adjusts ISO.
Practice exercises
- Aperture walk: photograph the same object at your widest and narrowest apertures and compare the backgrounds.
- Moving water: shoot a tap or fountain at 1/1000, 1/60 and 1/4 second.
- Noise test: photograph a dim room at every ISO step and see where the noise starts to bother you.
- One setting a week: spend a week in aperture priority, then a week in shutter priority.
Check your histogram as well as the screen, since a screen can make an image look brighter or darker than it is.
Post a photo where you changed one setting on purpose, or ask about a tricky lighting situation, in the comments.