Teleconverters are compact optical accessories that fit between the lens and the camera body, and effectively multiply the focal length of the lens they are fitted to. They are a less expensive way to extend your telephoto reach without having to purchase a super-telephoto lens which can be expensive. They are particularly useful for photographing wildlife or other subjects which would be scared off if working closely or in situations where you can't get close to your subject. Nikon currently offers the following teleconverters: 1.4x
Fit a 2x teleconverter on a 70-200mm zoom and you have an effective 140-400mm or a 1.5x converter on the same lens changes it to a 105-300mm. Not only do you get greater magnification or 'pulling power' but the lens's minimum focusing distance remains unchanged so a converter benefits close-up shooting too. However, depth-of-field does decrease.
The above image was taken at the 200mm end of a 70-200mm zoom lens. Adding a 3x teleconverter (not currently available from Nikon) to the lens, thus giving a focal length of 600mm, gets you a more tightly framed image without you having to move at all, as shown in the following shot.
On the face of it, it is all win-win with teleconverters. They are compact, lightweight, reasonably inexpensive gadgets that expand the range of focal lengths at your disposal.
However, there are important downsides. One, they are optical accessories so they can have a negative impact on image quality – resolution and sharpness can suffer and adding extra air/glass surfaces means flare can be more of a problem in backlit conditions.
Two, they cause a light loss which with some cameras means you need to keep an eye on shutter speeds to avoid camera shake and adjust the ISO accordingly. When using a 1.4x teleconverter you lose 1 stop of light while a 1.7x converter will lose 1 /12 stop of light. A 2x teleconverter loses two stops of light.
The light loss also means the viewfinder image may appear much darker than with the lens on its own. This makes composition trickier but it also means that the camera's autofocus system may be less efficient or it might struggle to work at all. However, with newer Nikon cameras you can still use the autofocus function when stopped down to f/8.
Generally speaking, you get the better optical performance with 1.4x or 1.5x teleconverters because their optical design is less complex and the 2x and 3x models should only be considered if you need the extra focal length. For general ‘just in case’ use, buy a 1.4x or 1.5x converter. Lenses of at least 100mm are recommended for 1.4x/2x converter use and 200mm for 3x models. It also makes sense to buy the best that you can afford.
Whatever you use, for the best possible image quality use the lens a couple of stops down from maximum aperture to ensure the best possible quality.
Finally, make sure that you use a decent monopod or tripod to ensure maximum camera stability too.