A lens device includes a control unit, an objective lens, an image sensor, a light source, a detecting unit, and a tube assembly. The tube assembly includes a guide barrel, an outer tube, and a lens holder. The guide barrel is disposed on the control unit. The outer tube includes a tube body that is rotatably sleeve don the guide barrel, and a guiding portion. The lens holder includes a holder body having a set of projections driven by the guiding portion. The image sensor is electrically coupled to the control unit. The objective lens is optically aligned with the image sensor. The light source is coupled electrically to the control unit. The detecting unit generates a detector output indicating angular orientation of the outer tube. The control unit is responsive to the detector output to control intensity of light outputted by the light source.