understanding exposure: aperture and iso

ISO measures the sensitivity of the image sensor. The higher the ISO on your camera is, the greater sensitivity it has to light. ISO controls how bright or dark your image will be. If you are shooting in daylight, you would want a low ISO so that your photo isn't overexposed. ISO affects the noise of your image. For example, the higher your ISO, the more light is captured, and this effect can make your image look gritty. The photo below of the lovely Devon shows the difference between having a low ISO and aperture vs. a high ISO and aperture.