I consider teleconverters to be an essential piece of photography gear for wildlife photography. Check out this great video on what makes them special and how to get the most out of them.
In this video, Steve Perry explains how teleconverters work, their effect on image quality and autofocus speed, and best practices to using them in the field. I use a 1.4x teleconverter paired on a 500mm f/4 lens a majority of the time. With quality glass, there should be little hesitation to add them on when the shot requires more reach.
One thing I wish was set straight here is how teleconverters affect depth of field. There’s enough conflicting information found from different sources that I wouldn’t mind a more scientific breakdown on it. Perry says that a 400mm f/2.8 with a 2.0x teleconverter would be an exact depth of field match to 800mm f/5.6. However, I’ve heard elsewhere, for example from Moose Peterson, that it would be more somewhere between f/2.8 and f/5.6. Using the PhotoPills depth of field calculator, it’s not an exact match, although it’s pretty close to the higher f-stop number. Commenters, feel free to flex your knowledge below.