The aperture refers to how big the iris is and how much light can enter the camera when taking a picture, it is measured in F-Stops ranging from F/1 all the way up to F/32, the photographer will set this setting depending on the Composition. the size of the iris (hole) within the lens. The bigger the hole, the more light gets into the camera. Aperture is measured in F-stops. A low number like F1.8 or F2.1 means a wider aperture. F16 would be a smaller aperture, A wider aperture (a low F-stop) means less of the image is in focus. It also means a faster shutter speed can be used.