Telekonverter

Teleconverter, also known as a tele extender, is an optical accessory placed between a camera body and lens to increase the effective focal length. By multiplying the lens’s focal length, it provides greater reach without changing lenses.

Most teleconverters are offered in magnifications such as 1.4x, 1.7x, and 2x. The result is a longer focal length and field of view corresponding to that factor, at the cost of additional light loss and a narrower maximum aperture.

A teleconverter contains fixed optical elements that pass through the image formed by the lens and extend its focal length. The attachment generally reduces the maximum aperture by about the same number of stops as the magnification (for example, a 2x converter typically reduces by two stops). Compatibility is important: teleconverters are designed for specific lens mounts and often for particular lens families; not all lenses work with all teleconverters.

Autofocus and metering behavior vary by system. Some camera–lens–teleconverter combinations support autofocus, but AF speed and reliability can degrade, and in some cases AF may be unavailable. Metering usually remains enabled but can be affected by the altered exposure.

Image quality can be affected by the combination. Teleconverters may soften detail, reduce contrast, and introduce aberrations, especially toward the edges at high magnification. They also can increase the minimum focusing distance and require higher ISO to maintain shutter speed.

Teleconverters are commonly used in wildlife, sports, and astronomy to extend reach when changing lenses is impractical. They are available as external accessories and, in some lenses, as built-in optical options.