 Unfortunately, with the lack of anything to prevent it, it also sucks in any airborne dust 

This lens is compatible with the Canon Extender EF teleconverters on newer EOS bodies.Autofocus works with the 1.4× Extender (and only with cameras that can autofocus at f/8) and image stabilization (IS) works with both 1.4× and 2× Extenders.The new Canon EF 100-400mm f/4.5-5.6L IS II USM was announced on 11 November 2014 (10 November in the United States due to time zone differences from Japan), with availability expected the following month.