A camera is an optical instrument that captures a visual image. At a basic level, cameras consist of sealed boxes, with a small hole that allows light through to capture an image on a light-sensitive surface. Cameras have various mechanisms to control how the light falls onto the light-sensitive surface. Lenses focus the light entering the camera. The aperture can be narrowed or widened. A shutter mechanism determines the amount of time the photosensitive surface is exposed to light.