A method and apparatus for the conditioning of articles. The zoom lens includes, sequentially from an object side thereof, a first lens elements group which has a negative refraction power as a whole, a second lens elements group which has a positive refraction power as a whole, and a third lens element group, which has a negative refraction power as a whole. The housing contains a belt drive mechanism including a plurality of belt pulleys engaged with the belt for receiving the conveyor belt from one of the upper and lower flight pads, applying motor power to the belt, and returning it to the other of the flight pads.