Can I use the Canon extender 1.4x with my ef 100-400mm f/4.5-5.6l IS II USM and 70D? Stephen Phillips, Devon

BRIAN SAYS… Adding a Extender 1.4x to any lens will reduce the maximum aperture by 1-stop. This makes your 100-400mm lens a 140-640mm lens with an f/6.3-8 aperture. Your EOS 70D’s AF system needs at least f/5.6 to operate so autofocus won’t work properly.