A teleconverter is an inexpensive lens supplement for detachable and fixed-lens cameras. Most commonly used with single-lens reflex (SLR) cameras, the device is designed to increase the magnification of any compatible lens. Due to the brand-specific design of lens mounts, the converter must also be brand-specific. The term "lens extender" is used by the Canon brand to refer to its line of teleconverters.
What a Teleconverter Does
A teleconverter optically enlarges the image coming through a lens. The effect is such that the magnification of a lens is multiplied by a certain factor.
How a Teleconverter Works
The teleconverter acts as another set of lenses behind whatever lens is being used. For SLR cameras, the converter is attached to the camera body and to the lens and has no individual focusing capability.
Types of Teleconverters
Because teleconverters attach to the lens mount, their manufacture is proprietary. Canon manufactures its extenders with standard magnification multipliers of 1.4x and 2x. Some other companies manufacture 3x converters.
Dimming
As with greater focal lengths, the use of a teleconverter reduces the amount of light reaching the film or sensor. For cameras with smaller apertures, this could necessitate much slower shutter speeds, and for cameras with in-body image stabilization, the compensation may be insufficient.
Autofocus
Even with the use of a compatible teleconverter, auto-focus may become slowed or inactive.
Price
The biggest advantage of teleconverters is price. While the cost of a whole lens is often less than the equivalent lens and converter, it is almost always less expensive to purchase a teleconverter than to purchase a new lens.
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