Section 2 aperture The aperture opening on a camera works in the same way as the pupil of an eye. If there is too much light coming in then the size of the aperture will be reduced, and if the light is dim then it needs to be increased in order to let more light in. This calculation is determined by the f-stop. The f-stop values displayed on the lens are fractions of its focal length. The terms `open up' and `stop down' are used to increase or decrease the diameter of the aperture. The size of the aperture will also dictate the depth of field. Depth of field (or depth of focus) is the total distance in front of and behind your picture's main subject whilst it remains in sharp focus. This depth (or length) of field is not what you are focusing on, but it does create a sense of distance in the final image. Understanding and controlling depth of field is a great creative tool to consider and apply when creating interesting images.