A lens device includes a lens barrel, a lens, a guiding unit, and a positioning unit. The lens barrel has a barrel axis. The lens has a lens axis and is disposed between the lens barrel and the positioning unit. The guiding unit includes guiding studs and guiding slots formed on the lens barrel and the lens, wherein the guiding slots receive the guiding studs respectively therein and have dimensions larger than those of the guiding studs. The positioning unit has outer and inner ring sections. The outer ring section has a ring axis and is sleeved on the lens barrel. The inner ring section engages the lens such that the lens axis forms a radial offset with the ring axis, wherein the lens is positioned relative to the barrel axis by the inner ring section according to angular orientation of the inner ring section relative to the ring axis.