A lens apparatus includes an optical system having an aperture unit and a plurality of extender lenses that are selectively locatable in an optical axis. The lens apparatus is mountable on a camera having an image pickup element. To maintain the brightness of the image when changing the extender lens, the lens apparatus has a memory in which correction data associated with aperture values for each extender lens, and the aperture unit is driven using the correction data according to information on the extender lens.