You can increase the focal length of almost any lens by using an extender. An extender (formerly called a tele-converter) is a lens set in a small barrel and then mounted between the camera body and a regular lens. Canon offers two extenders, a 1.4x and 2.0x, that are compatible only with certain L-series Canon lenses. Extenders don't affect the MFD (Minimum Focus Distance) of the lens they're mounted behind, thus the MM (Maximum Magnification) of the lens is also multiplied by 2x. Extenders can also be stacked to get even greater magnification.
For example, using the Canon EF 2x II extender with a 600mm lens doubles the lens's focal length to 1200mm. Using the Canon EF 1.4 II extender would increase a 600mm lens to 840mm. However, a noticeable trade-off occurs in quality over using a lens specific to that particular focal length. In other words, you generally get better results from a 400mm lens than with a 200mm lens plus the Canon EF 2x II extender. The native lens is also less trouble to mount and dismount.
Extenders generally don't change camera operation in any way, but they do reduce the amount of light transmission reaching the image sensor. The EF 1.4 II extender decreases the light by 1 f-stop, and the EF 2.0 II extender decreases the light by 2 f-stops and you lose a little autofo-cusing speed. In addition to being lightweight, the obvious advantage of extenders is that they can reduce the number of telephoto lenses you carry.
The 1.4 II extender can be used with fixed-focal-length lenses of 135mm and longer (except the 135mm f/2.8 Softfocus lens) and these zoom lenses:
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