Accessory Lens Set, Basic Optics An accessory for PASCO's Basic Optics Systems, the Accessory Lens Set includes +250 and -150 mm focal length lenses. Each lens is mounted in a lens holder for protection and easy storage. Applications: Construct a telescope Create and study virtual images The Accessory Lens Set -- Basic Optics (OS-8519) is an accessory for the: Basic Optics System (OS-8515C) Basic Optics System II (OS-8546B)