From the B&H YouTube Channel:
The word on the street is that you simply cannot make sharp images with teleconverters (TCs) especially with the 2X. Nothing could be further from the truth. With some practice and good sharpness techniques you can learn to use TCs effectively to photograph small, distant, or shy subjects.
Teleconverters or multipliers come in a variety of strengths, most usually 1.4X and 2X. Nikon also offers a 1.7X TCE. Each will multiply your effective focal length by the factor in its name. A 1.4X TC will make your 400mm lens into a 560mm lens. A doubler, aka a 2X TC, will enable your 600mm lens to give you 1200mm of reach.
In this program, Arthur Morris, internationally noted bird photographer and educator, will share the tips and secrets that he has developed and used for more than three decades so that you too can use TCs to improve your photography. This program will be well-illustrated with Artie’s spectacular images of birds, wildlife, flowers, and even a few unexpected subjects.
B&H carries teleconverters.