A lens includes a central optical portion, a peripheral portion surrounding the central optical portion, a first ZnS coating formed on the central optical portion and the peripheral portion at one side of the lens, and a first MgF2 coating formed the central optical, the peripheral portion being free of MgF2 coating formed thereon. The optical thickness of the first ZnS coating is one quarter of a wavelength of predetermined light passing therethrough. The optical thickness of the first MgF2 coating is one quarter of the wavelength of the predetermined light.