When the shutter-release button is pressed, light is allowed to reach the sensor and a photograph is produced. Three things control how much light reaches the sensor: The amount of time the shutter is open (shutter speed); how much light is allowed to pass through the lens (aperture); and how sensitive the camera sensor is to light (ISO).
Using these three ways to control the amount of light that reaches the sensor can produce the look you want in your images. If too much light reaches the sensor, the image is too bright, and the details in the bright parts are lost. This is called an overexposed image. When too little light reaches the sensor, the image is too dark and the details do not appear in the dark areas. This is called an underexposed image. The goal is to allow enough light to reach the sensor so that there is detail in both the bright and dark areas.
It’s important to understand standard terminology about exposure. The shutter speed is described using a measure of time—most often in fractions of a second. For example, for Figure 6.1, the shutter speed was 1/250 second. Aperture is described using the term f-stop. For Figure 6.1, the f-stop was f/8.0, which describes the size of the camera’s aperture during the 1/250 second the shutter was open. The ISO has a standard numerical value that is the same across all cameras.
6.1 This photo has detail in both ...