Teleconverter

A teleconverter, also known as an extender, is an accessory lens that is placed between a camera body and a interchangeable lens. Its primary function is to magnify the image projected by the main lens, effectively increasing its focal length. This is achieved by a system of internal lenses that relay and enlarge the image cone coming from the primary lens. For example, a 1.4x teleconverter will increase the focal length of a 200mm lens to 280mm, and a 2x teleconverter will double it to 400mm.

Teleconverters have the advantage of being a more affordable and compact way to achieve longer focal lengths compared to purchasing a dedicated telephoto lens. They are particularly useful for wildlife, sports, and other photography where reaching distant subjects is essential. However, this magnification comes at a cost. Teleconverters reduce the amount of light that reaches the camera's sensor, typically by one or two stops depending on the converter's magnification factor. This means that autofocus performance can be slower, and image quality may be slightly degraded, especially in terms of sharpness and contrast, compared to using the primary lens on its own. The effectiveness of a teleconverter can also vary depending on the quality of both the teleconverter and the primary lens it is used with.