Photography basics
The exposure triangle, without the jargon
Aperture, shutter speed and ISO all affect brightness, but each also changes how a photograph looks. The useful skill is not memorising three definitions; it is knowing which decision to make first.
The short version
- Choose aperture when depth of field matters most.
- Choose shutter speed when movement matters most.
- Use ISO to make the first two choices possible in the available light.
Aperture: how much light, and how much looks sharp
The aperture is the opening inside the lens. A lower f-number, such as f/2.8, means a wider opening. It lets in more light and can produce a softer background. A higher number, such as f/11, lets in less light and usually keeps more of the scene acceptably sharp.
That does not make a low or high number automatically better. For a portrait you may want separation from the background. For a landscape, group photograph or close-up, you may need more depth. Your distance from the subject and the lens you use also affect the result.
Shutter speed: freeze movement or let it show
Shutter speed is the length of time the camera records light. A fast speed such as 1/1000 second can freeze quick action. A slower speed such as 1/15 second records more movement—both from the subject and from your hands.
A common starting point for a stationary subject is a shutter speed at least as fast as the reciprocal of the focal length: around 1/100 second with a 100mm lens, for example. Treat that as a starting point, not a law. High-resolution cameras, subject movement, technique and image stabilisation all change what is safe.
ISO: the practical compromise
ISO controls how strongly the camera amplifies the signal it records. Raising it helps you use a faster shutter speed or a smaller aperture in low light, but it can add visible noise and reduce the amount of highlight and shadow detail the file holds.
Use the lowest ISO that still lets you make the photograph you want. Do not be afraid to raise it when the alternative is unwanted blur. A sharp, slightly noisy photograph is often far more useful than a clean but blurred one.
How to choose the settings in the right order
Start with your intention. If background softness matters, choose the aperture. If a child, animal or cyclist is moving, choose the shutter speed. Then raise or lower ISO until the camera can give you a workable exposure.
Aperture Priority is often the easiest mode for people and still subjects. Shutter Priority is useful when movement is the main problem. Manual mode becomes helpful when the light is consistent and you want the exposure to stay unchanged from frame to frame. Auto ISO can still do useful work in any of these situations.
- Portrait: choose the depth of field, then make sure the shutter is fast enough for the person.
- Moving child or pet: choose a shutter speed that stops the motion, then accept the ISO the light requires.
- Handheld landscape: choose enough depth of field, check the shutter is safe to handhold, then set ISO.
A ten-minute exercise
Place one object near a window with something several metres behind it. Take one photograph in Auto as a reference. In A or Av mode, make three photographs at a low, middle and high f-number. Then try S or Tv mode and photograph a moving hand at 1/30, 1/125 and 1/500 second.
Review the pictures alongside their settings. Look at the background, the moving hand and any change in noise. This short comparison teaches more than trying to remember a chart in isolation.