I have a Canon 6D along with a 70-200mm Lens. I occasionally need something that can zoom closer. For the amount I`d use it I don`t think I could justify a 400mm + lens. Would an extender do a similar job? I understand there`d be a loss of speed and perhaps a tad less sharp but has anyone else used one of these rather than buy a lens with a longer zoom and can you recommend a decent one?