Aperture controls light and depth of field

Aperture is the opening inside the lens, written as an f-number. Moving from f/4 to f/2.8 opens the lens by one stop and lets in roughly twice as much light for the same shutter speed. Moving from f/4 to f/5.6 halves the light. A smaller f-number means a wider opening.

A wider aperture can also give shallower depth of field, leaving less of the scene acceptably sharp. Distance to your subject, focal length and sensor format matter too. An f/1.8 portrait made close to a person is a different focusing challenge from a distant landscape at the same setting. For a group in more than one row, start with more depth of field and check the people at the front and back.

Shutter speed decides how movement looks

Shutter speed is the length of the exposure. At 1/500 second, each exposure is brief enough to hold many everyday movements sharp; at 1/30 second, the same movement may become a blur. These are starting points, not guarantees: a fast subject nearby can need a much shorter exposure.

Image stabilisation helps with movement of the camera. It does not freeze a person walking, laughing or dancing. If a face is blurred but a chair beside it is sharp, increasing stabilisation is unlikely to solve the problem. Try a faster shutter speed.

ISO helps set image brightness

ISO affects how captured light is translated into the brightness of the recorded image. Increasing ISO does not make more light reach the sensor. A higher setting can let you keep a faster shutter speed or a smaller aperture in low light, but noise and highlight headroom can become more noticeable limitations.

A sharp photograph at a higher ISO is often more useful than a beautifully clean file with a blurred face. Avoid judging noise only by a very large view on screen; also look at the intended print or delivery size.

Work through one indoor portrait

Imagine you have a correct exposure at f/4, 1/125 second and ISO 1600, but your subject keeps moving. Changing to 1/250 second halves the light. You could raise ISO to 3200 to keep roughly the same brightness, or open to f/2.8 and keep ISO 1600. The first choice trades some image quality for motion control; the second trades some depth of field.

If the person’s eyes are on different planes or you are photographing two people, the wider aperture may be the wrong compromise. Add light, move nearer a window, or accept the higher ISO. There is no single setting that makes all three trade-offs disappear.

A useful order for your next shoot

Start with the subject, then make one adjustment at a time. This makes it much easier to understand why a photograph improved.

  • Choose a shutter speed that suits the movement.
  • Choose an aperture that gives enough depth of field.
  • Use ISO or additional light to achieve the brightness you need.
  • Check a face at a useful magnification, then check important highlights.
  • When the light changes, repeat the check rather than assuming yesterday’s settings still fit.