Teleconverter
A supplementary optic mounted between lens and body that multiplies focal length at the cost of light and, usually, sharpness.
Formula / rule
N_new = N × magnification
Measured in
× magnification
In depth
A 1.4× converter costs one stop and a 2× costs two, because the same cone of light is spread over a larger image. Optical quality depends heavily on the parent lens: a converter magnifies its flaws along with its focal length, so it pairs well with high-grade telephotos and poorly with kit zooms. Autofocus may also slow or disengage once the effective aperture passes f/8.
Examples in practice
A 300 mm f/2.8 with a 2× converter becomes a 600 mm f/5.6 — half the price and weight of a dedicated 600 mm.