I now use the Canon lens 400mm F5.6 L USM. That is a very sharp lens even wide open and even wide open with a 1.4x and this is why I use it, because every pixel count when doing wildlife and bird photography. Eventualy? I will move to a 500mm F4 (dreaming), but even then I would keep my 400mm F5.6 L lens because it is small and portable compared to the 500mm which is very big and heavy. For long hikes, there is nothing like a small portable lens with good quality and a small monopod.