Aperture
APERTURE or F-STOP
The aperture also known as the f-stop controls the intensity of light that is allowed to pass to the sensor.
The most commonly used aperture control device is the iris diaphragm. An iris diaphragm is an adjustable device that is fitted into the barrel of the lens or shutter housing. It is called an iris diaphragm because it resembles the iris in the human eye. The size of the aperture is changed by turning the aperture control.
When you decrease the aperture size by one full f/stop the intensity of the light striking the sensor will be cut in half. If you increase the aperture size by one full f/stop you will double the intensity of the light striking the sensor.
In Addition to controlling the light intensity, the aperture also allows you to control Depth of Field (DoF). Depth of field refers to the zone both in front of and behind your subject that is in acceptably sharp focus. You will learn more about Aperture and Depth of Field in the Camera Controls section of this Photography 101 Course.