"When taking a photograph, there are really only three controls that
matter on a camera: focus, shutter speed and aperture. Focus is
reasonably obvious; you can quickly spot when a picture is out of focus.
Likewise shutter speed is easy to understand since it is measured in
fractions of a second. However aperture is a mystery to most people,
because it is usually explained using technical jargon and its effects
are not immediately obvious."