A lens apparatus includes an imaging optical system including a focus lens unit, a branching optical unit disposed at the image side of the focus lens unit, a focus-state detecting unit including a focus detecting element having a focus detection area that receives light beams from the branching optical unit, and detecting a focus state by using the focus detecting element, an actuator that drives the focus lens unit in an optical axis direction thereof, a controller that controls the actuator, in accordance with an output from the focus-state detecting unit, and a movable image-side optical unit disposed at the image side of the branching optical unit. The controller changes the size of the focus detection area of the focus detecting element within a range in accordance with the position of the image-side optical unit.