The aperture is the opening in the lens (created by an adjustable iris or diaphragm) that allows light to pass through. The exposure of the image is determined by the combination of shutter speed and the opening of the aperture. The larger the aperture, the more light is allowed to pass through the lens. The aperture is measured in f-stops, and each stop represents a factor of two in the amount of light admitted. The aperture setting (f-stop), combined with the focal length of the lens, determines the depth of field of an image. For more information on depth of field.