Exposure: Taking a good photo requires getting the right amount of light to the sensor or film. Aperture: Diameter of lens opening. Expressed as fraction of focal length (called f-number). A big f-number means stopped down aperture. Exposure: The same exposure can be obtained with a shutter interval twice as long and aperture area half as big. Depth of field preview: When using “aperture priority” mode, the camera still uses the widest aperture for the view finder.