i have sold my tamron 200-500, i am left with canon 400mm... do i have to invest in lens with shorter focal length (like 70-300 or 70-200,f-4 IS +1.4 tc) or it is better to wait and go for even longer lens? ( i have seen many excellent images with lens like 500,f-4),... how about addition of 'kenko pro 1.4 300 DG' to my 400,f-5.6?