There are two lenses that do work well with a 1.4x tel-con. Never the 2x however. The ef 70-200mm f2.8L and the ef 300mm f4l IS. The nice thing about the 300mil is it has the IQ to give up, but it becomes a 4200mm f5.6 lens with IS. Canon's fantastic and unique ef 400mm f5.6L does not have IS. So the converted 300mil is a good give and take.