Exposure triangle
The three-way tradeoff between aperture, shutter speed, and ISO: change one for brightness, then pay for it with focus, motion, or noise.
See it
What it is
The exposure triangle is the working relationship between aperture, shutter speed, and ISO. Aperture changes how much light the lens admits, shutter speed changes how long it lands on the sensor, and ISO changes how the captured signal is rendered. Move any one by a stop and you can move another by a stop in the opposite direction to hold the same brightness.
Reach for it in manual mode when brightness is only half the decision. Choose the visual constraint first: shutter speed for motion, aperture for depth of field, then ISO to place the exposure. A runner might force 1/1000 second; a group portrait might force f/8. The triangle tells you where the remaining light has to come from.
Gotcha: the corners are not interchangeable in what they do. A wider aperture can blur the background, a slower shutter can blur the subject, and higher ISO can expose noise. On a digital camera only aperture and shutter change how many photons are captured, so the triangle is a decision tool, not a literal account of sensor physics. It is a teaching model popularized by Bryan Peterson's book Understanding Exposure, not a law of optics.
Ask AI for it
Set this sports photograph with the exposure triangle in manual exposure mode: lock shutter speed at 1/1000 second to freeze the runner, choose f/4 to keep the face and torso sharp while separating the stands, then set ISO 1600 to land the exposure. Render a clean, correctly exposed frame with no motion blur and no simulated camera shake.