The opening which adjusts the diameter of the group of light rays passing through the lens. In interchangeable lenses used with single lens reflex cameras, this mechanism is usually constructed as an iris diaphragm consisting of several blades which can be moved to continuously vary the opening diameter. With conventional SLR camera lenses, the aperture is adjusted by turning an aperture ring on the lens barrel. With modern camera lenses, however, aperture adjustment is commonly controlled by operating an electronic dial on the camera body.
Canon glossary EU