There are two devices in your camera that control the amount of light reaching the sensor/film. The first I'm going to cover is the aperture. The aperture (also know a F/stop) is a device in your lens that creates a hole which light passes through. It's measured in numbers (2.8, 3.5, 4.0, 5.6, 8.0, 11....) the SMALLER the number the LARGER the hole. The primary thing that aperture controls is depth of field (area of focus). A large hole creates a shallow area of focus, a small hole creates a deep area of focus. The image on the left was shot at F/22 bringing the entire image into focus. The image on the right was shot at F/2.8 forcing me to choose about a 2 foot area to bring into focus.
