Extender EF 1.4x
The Extender EF 1.4x has been developed to expand the use of the L telephoto lenses. When fitted, it provides an equivalent lens performance to a prime lens on its own.
By using convex lenses of glass with a low index of refraction and concave glass lenses with a high index, the deterioration due to astigmatism common to extenders has been corrected, as has chromatic aberration by balancing both types of lens in the first group.
Spherical aberration and astigmatism have also been corrected by the use of a convex lens with weak refractive power in the rear group.
As the F number only drops by 1 stop when the 1.4 extender is used, AF photography is still possible even with the EF 600mm f4.0L and the viewfinder image brightness remains good.