The Fotodiox 2x Teleconverter for Canon cameras is a precision optical component designed to expand your photographic capabilities. This premium quality 2x teleconverter, effectively doubles the focal length of the lens it's used with and maintains the resolution and contrast of any fine optic. The teleconverter will multiply the focal length of the existing lens by 2x, and will cut back the amount of light by 2 f/stops. For example, using a 2x teleconverter with a 300 mm lens will effectively increase the focal length to 600 mm and a lens set to f/4 will now be f/8. A maximum aperture of f/4 or greater is required in order to fully utilize the autofocus capabilities. Please be aware that AF will work properly only if there is enough light and contrast on the subject to activate the camera's AF sensors. Manual focusing is recommended when using lenses with a smaller open f-stop value than those given above.