Nikon TC-20EII Tele extender.

The Nikon TC-20E III 2x Teleconverter is an exceptional teleconverter optimized for use with FX and DX digital SLR cameras. It doubles the focal length of compatible Nikkor lenses. The TC-20E III is constructed of 7 elements in 5 groups and with Nikon's Integrated Coating (IC) you will see improved color and reduced flair. It has one aspherical lens element which virtually eliminates lens aberrations. The 2x extender (also known as a teleconverter) will double the focal length of your existing lens, and will cut back the amount of light by 2 f/stops.