For situations where light levels change over the course of time, such as outdoor installations, rooms with exterior windows or offices with different light levels at night, auto-iris lenses should be used. They have a motor attached to the lens that automatically adjusts the aperture (iris) of the lens to correct for changing light levels. Auto-iris lenses come in two different varieties: Video Drive (VD) and Direct Drive (DD). With VD lenses, the lens itself contains the necessary circuitry to control the iris. With DD lenses, the amplifier circuitry is not present so the camera must supply it. Because DD lenses do not have the iris control circuitry, they are cheaper, and since most security cameras now support both VD and DD lenses, DD lenses are the right choice in most circumstances.