A lens assembly for a mobile device includes an adjustable diaphragm device capable of being combined to a first lens unit, or between the first lens unit and a second lens unit. The adjustable diaphragm device includes a case having a light transmission hole being communicated with an inner space of the case, a diaphragm sheet with an aperture, and a driving mechanism disposed in the inner space of the case. The driving mechanism includes a driving member and a swing arm. One end of the swing arm is movably connected to the case and the opposite end thereof is fixedly connected to the diaphragm sheet. The driving member is movably connected to a position between two ends of the swing arm. The driving member is controlled by a control unit to move the swing arm and thus the diaphragm sheet, in order to control the amount of light.