A teleconverter is an optical component that fits between camera and lens and increases the focal length without altering the minimum focusing distance. They are most commonly made in l.4x and 2x versions - although l.7x and 3x versions are also produced. Therefore, a 200mm telephoto combined with a l.4x converter would be equivalent to 280mm, while the same lens coupled with a 2x multiplier would be transformed into a 400mm lens. Converters are relatively inexpensive, light and compact, so are a convenient and economical way to expand the overall flexibility of your kit, making them a great choice for nature photographers in particular. However, there are disadvantages to using converters.

Firstly, they reduce the amount of light entering the camera by one stop (l.4x) or two stops (2x) respectively. This can prove a problem in low light, when the risk of camera shake is increased, and when photographing fast action, when subject blur is more likely due to the resulting slower shutter speed. The other drawback of attaching one is that image quality is slightly degraded, with zoom lenses being affected more than fixed focal lengths. They can also lead to problems with autofocus too.

Generally speaking, though, the benefits of using one outweigh the disadvantages. For best quality, opt for one with more elements in the construction. Nikon, Canon and Sigma are among the camera manufacturers with teleconverters in their range, but check compatibility as some are designed for use with specific lenses. Look at independent brands like Tokina, Tamron and Kenko for teleconverters with more universal compatibility - your local photo store can offer expert advice.