It increases the focal lenght by 1.4X. Its 5 elements in 4 groups suppress aberations and preserve the prime lens' optical performance. With the Extender attacthed, the maximum aperture of the prime lens decreases by only one stop. Autofocusing is enabled with prime lenses having a maximum aperture of F4* or larger.
When used with the EOS-3, autofocusing is possible at the center focusing point up to F5.6 with the EF 1.4X (excluding the EF 180mm F3.5L Macro USM).