A camera offers a lot of settings. Three are a useful place to begin: aperture, shutter speed and ISO. Instead of memorizing one perfect combination, start with the photograph you want to make. Does the background need to soften? Does movement need to stop—or leave a trace?
Aperture: how wide the lens opens.
The aperture controls the size of the opening through the lens. A lower f-number means a wider opening and more light at the same shutter speed. It can also produce a shallower depth of field, though focal length, subject distance and format matter too. Think of it as a choice about both exposure and how much of the scene appears acceptably sharp.
Shutter speed: how time enters the picture.
Shutter speed determines the duration of an exposure. Shorter exposures can freeze movement; longer ones can reveal it as blur or a trail. A fast-moving subject, camera movement and the direction of travel all affect what looks sharp. The trail photograph above illustrates movement, rather than prescribing the unverified settings used to make it.
ISO: brightness with a tradeoff.
Digital ISO influences how the camera renders the captured signal into an image. Raising it can help you keep a needed shutter speed or aperture in dim light, but image noise and highlight latitude deserve attention. ISO does not open the lens wider or extend the exposure time.
Choose one priority, then balance the rest.
For a moving subject, begin with a shutter speed that gives the motion treatment you want. For a portrait, decide how much background detail belongs in the frame. Make a test photograph, inspect important highlights and sharpness, then adjust. No single setting is a guarantee; the light and the subject get a vote.