This article discusses the feasibility of using the Canon EF 1.4x II teleconverter on Canon lenses that are listed as being non-compatible with it. Canon states that the 1.4x and 2x teleconverters will not work on any prime lens under 135mm nor any zoom lens with a lower focal length less than 70mm such as the 28-300mm L. Why not? Physics dictates that it should work since everything is in the same plane. A lot of sites state that it is an issue with auto-focus or metering, but with manual focus and ETTL, this should be a non-issue, right? Not exactly, before you try to mount any of the teleconverters on a lens that is stated as being incompatible read on.