200mm f2.8 is important for u can add in 2x teleconvertor, become 600mm f5.6, auto focus still function in this range.
But if 300mm f5.6, after a 1.4x become 420mm f11 or after a 2x teleconvertor become 900mm f22, auto focus not working anymore.
from this somewhere from a book, not very sure since i dont have teleconvertor :sweat: :bsmilie:
build quality also a problem, f2.8 is more sharp.....
i have a canon 75-300mm f4-5.6 cost less than $300
but a 70-200mm f2.8L cost more than 1k
a 70-200mm f2.8L IS USM need about 3k :sweat: