In photography and optics, aperture refers to the opening in a lens through which light passes to enter the camera or optical system. It is expressed in f-numbers like f/1.4, f/2, f/8, where a smaller f-number means a larger aperture and a larger f-number means a smaller aperture. Aperture controls the amount of light entering the camera and affects exposure, playing a fundamental role in determining the optimal exposure in an image, along with ISO and shutter speed. Aperture can also add dimension to photos by controlling depth of field, which is the range of distance in an image that appears to be in focus. A larger aperture (smaller f-number) creates a shallow depth of field with only the subject or a portion of the subject in focus, while a smaller aperture (larger f-number) creates a deeper depth of field with more of the image in focus. Aperture priority is a semi-automatic shooting mode used in cameras that allows the photographer to select an aperture setting and let the camera decide the shutter speed and sometimes also ISO sensitivity for the correct exposure.