Because shutter speed and aperture numbers are doubled or halved of the next numbers on those scales, you can cause the same amount of light to enter your camera at more than one combination of aperture and shutter speed settings. For example, a properly exposed picture under certain conditions may require a shutter speed at 1/250th and an aperture at f/22. (This could be on a very sunny day.) You could also set your shutter speed at 1/500th and your aperture at f/16, and also expose the picture correctly.
