The Sigma TC-2001 2x Teleconverter multiplies the focal length of Nikon mount lenses by 2 while still retaining full communication to enable autofocus to operate on compatible lenses at f/8.
The TC-2001 2x teleconverter also features two Special Low Dispersion element to reduce chromatic aberrations, and features a splash and dust proof construction for improved durability.

Teleconverters are great for improving reach, and also increases the flange distance between the lens and body which decreases the minimum focus distance for even closer macro capabilities.