The Sigma TC-2001 2× Teleconverter extends the focal length of your lens. This teleconverter is made for Nikon F-mount lenses and cameras, and lets the lens communicate with the camera to allow an autofocus of up to f/8. There are two Special Low Dispersion lenses in the optical design of 10 elements in 5 groups that reduces chromatic aberrations. The light loss is limited to two stops for maintaining the image quality and ensuring accurate focus and exposure adjustments. You may use the teleconverter in challenging weather conditions, thanks to its dust and splashproof construction.