An embodiment of the invention provides a focus adjustable apparatus adapted to be disposed on a transparent substrate to modulate a plurality of rays passing through the transparent substrate. The focus adjustable apparatus includes a light modulation device, an eye tracking device, an eyesight status device and a controller. The light modulation device receives the rays from the transparent substrate and adjusts an emergent angle of each ray. The eye tracking device tracks a position of a user and estimates a distance between the user and the focus adjustable apparatus. The eyesight status device obtains an eyesight data of the user. The controller provides a control signal to drive the light modulation device by estimating the emergent angle of the each ray according to the distance between the user and the focus adjustable apparatus and the eyesight data of the user.
