A camera is an optical instrument that captures a visual image. It is a light-tight box with an aperture to admit light focused onto a sensitized film or plate. The aperture can be narrowed or widened. Cameras have various mechanisms to control how light falls onto the light-sensitive surface. It has lenses that focus on the light entering the camera. A shutter mechanism determines the amount of time the photosensitive surface is exposed to light. It is the main instrument in photography.