When you take a photograph, the exposure is determined by the area of the aperture and the exposure time (also called shutter speed). Although exposure is basically quantity-less, there are a range of combinations of aperture and exposure time that will create a good photographic exposure. If the aperture is too wide or the exposure time too long, then all you’ll get is a white photo; conversely, if either of them is too low, you’ll just get a black photo.