Every photo's brightness comes from three settings working together. Change one and you usually change another to keep the exposure right. Each has a side effect, and choosing which side effect you want is what makes a photo look the way it does.
Aperture: how wide the lens opens
Aperture is written as an f-number. A smaller number means a wider opening and more light: f/1.8 lets in far more light than f/8. The full-stop sequence is f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11 and f/16, and each step halves the light. The side effect is depth of field: wide apertures blur the background, narrow ones keep more of the scene sharp. See depth of field explained.
Shutter speed: how long the sensor sees the scene
Fast speeds such as 1/1000s freeze sport and wildlife; slow speeds such as 1/30s let in more light but blur anything moving, including your own hand shake. A rule of thumb for hand-holding without stabilisation is at least 1 over the focal length (allowing for crop): 1/100s at 100mm on full frame, or 1/160s on APS-C. Stabilisation lets you go slower for still subjects but doesn't freeze moving ones.
ISO: how much the signal is amplified
Raising ISO brightens the image but adds noise. As a rough guide, modern full-frame cameras stay clean to around ISO 6400, APS-C to around 3200 and Micro Four Thirds to around 1600, though noise reduction software has pushed these limits up.
Putting it together
- Kids indoors: f/2, 1/250s, auto ISO.
- Landscape on a tripod: f/8, ISO 100, whatever shutter speed the light needs.
- Football on a sunny day: f/5.6, 1/1600s, auto ISO.
- Northern lights: f/2.8, 5 seconds, ISO 3200.
Which mode to use
Aperture priority (A or Av) lets you choose the aperture while the camera picks the shutter speed. Many experienced photographers use manual mode with auto ISO: you set aperture and shutter speed, and the camera sets ISO. Use exposure compensation to make the result brighter or darker.