|Canon teleconverter lenses
Teleconverters are compact units which fit between a lens and camera body to multiply the focal length. A 2x teleconverter will multiply a lens focal length by two times, allowing, say, a 200mm lens to deliver the same field of view as a 400mm. This saves you from buying or carrying two lenses.
Teleconverters can work with almost all lenses, but are designed to work best with telephotos. Note a handful of lenses may not be compatible with teleconverters, so check before buying.
The downside to teleconverters is a slower focal ratio. If you use a 2x teleconverter, both your focal length and your smallest f-number will double. So if you had a 200mm f2.8 lens, it would become a 400mm f5.6 lens. This represents two stops difference, which means an exposure four times slower. A slower focal ratio can also cause problems for some autofocus systems which tend to only work at f5.6 or brighter. So if you use a 2x teleconverter on an f5.6 lens, it will become an f11.2 model with manual focus only.
Canon offers a 2x and a 1.4x teleconverter. The 1.4x model clearly won’t extend the focal length as much, but only results in the loss of one stop – so you’ll only need an exposure twice as long. A 200mm f2.8 lens with a 1.4x teleconverter would therefore act like a 280mm f4 model.
For a full explanation of lens specifications and examples of coverage at different focal lengths, check out our Camera Labs Lens Buyers Guide.