Photography
The exposure triangle: aperture, shutter speed and ISO
By Courses4Anyone · Published
Every photograph is a balance between three settings that control how much light reaches the sensor and what the picture looks like. Once you understand how aperture, shutter speed and ISO trade off against each other, automatic mode stops being a mystery and manual choices become practical decisions instead of guesses.
How the three exposure settings work together
What does aperture control?
Aperture is the size of the opening in the lens, written as an f-number such as f/1.8 or f/8. A smaller number means a wider opening, more light and a shallower zone of focus, which is why portraits often use f/1.8 to f/2.8 to separate a person from the background.
A larger number means a narrower opening, less light and more of the scene in focus, which suits landscapes and architecture. Very small apertures can also soften an image slightly through diffraction, so the sharpest setting is often in the middle of a lens's range.
What does shutter speed control?
Shutter speed is how long the sensor collects light, from fractions of a second to several seconds. Fast speeds such as 1/1000 s freeze motion; slow speeds such as 1/4 s let movement blur, which can look deliberate for flowing water or light trails.
For handheld shots, a common starting rule is to keep the shutter speed at least as fast as one over the focal length, so a 50 mm lens works at about 1/50 s or faster. Image stabilisation and steady technique can relax that rule, but moving subjects still need a faster speed than the rule suggests.
What does ISO do, and when should you raise it?
ISO amplifies the signal from the sensor. A low value such as ISO 100 gives the cleanest image in good light. Raising it brightens the picture without changing aperture or shutter speed, but it adds visible noise, particularly in shadows.
Raise ISO only after you have opened the aperture and slowed the shutter as far as the subject allows. A slightly noisy sharp photo is usually more useful than a clean blurred one.
How do you choose which setting to prioritise?
Decide what matters most in the frame, set that first, then let the other two compensate:
- — Moving subject: choose the shutter speed first, then open the aperture, then raise ISO.
- — Background separation or depth: choose the aperture first, then adjust shutter speed and ISO.
- — Dark scene on a tripod: keep ISO low and use a longer shutter speed.
- — Check the histogram after each test frame so you can see clipped highlights or crushed shadows.