In days past, a photographer would measure the level of light at a location with a light meter
and then calculate the best combination of shutter speed and aperture to correctly expose the film.
This is now done automatically by the camera. Aperture is the measure of how much light is
passing through the lens. The lens has an iris, which can be 'stopped down' to reduce the amount
of light passing through the lens. A fully open lens will pass the greatest amount of light - full
aperture - but this also reduces the 'depth of field'. The depth of field is the band of distance over
which the subject is in focus. In 1 above, with full aperture either your girl friend or the harbour is
in focus - but not both. By 'stopping down' the lens - reducing the aperture - both can be brought
into focus. But as a consequence the amount of light passing through the lens is reduced. The
length of time which the shutter is open will have to be increased to compensate.