Abstract: A head-up display with variable focal length includes a projector which provides a beam with image, a magnifier lens module which is disposed on the route of the beam, and an array of lens which is movably disposed between the projector and the magnifier lens module. The beam forms an inter-image after passing the array of lens, then forms a virtual image after passing the magnifier lens module. To change the position of the virtual image, adjust the relative position between the array of lens and the magnifier lens module. A dispersion angle of the array of lens is ?d, a magnification of the magnifier lens module is M, a distance between the virtual image and user's eyes is VID, a visible range of user's eyes is Et, and satisfies the relationship as following: ?d=M*2*cot?1 (2VID/Et).

Abstract: A head-up display with variable focal length includes a projector which provides a beam with image, a magnifier lens module which is disposed on the route of the beam, and an array of lens which is movably disposed between the projector and the magnifier lens module. The beam forms an inter-image after passing the array of lens, then forms a virtual image after passing the magnifier lens module. To change the position of the virtual image, adjust the relative position between the array of lens and the magnifier lens module. A dispersion angle of the array of lens is ?d, a magnification of the magnifier lens module is M, a distance between the virtual image and user's eyes is VID, a visible range of user's eyes is Et, and satisfies the relationship as following: ?d=M*2*cot?1 (2VID/Et).