Originally posted by Pål Jensen Yhis is either a zoom lens or a prime as long or longer than 400mm
I agree with this conclusion. A long zoom such as a 135-600 not intended for use with a converter, would seem likely or a rebirth of the 600 f4 but the latter
is very expensive.
by way of illustration of the issue, I own a 300mm f2.8 and a 600mm f5.6.
the 600 really doesn't do much for me. IT is vastly heavier and more difficult to use than the 300mm, and since I have also a 2x-L converter for my300 F 2.8 and produce similar results.
While you can argue that the 600 f/5.6 may produce better results than the
300 augmented with a 2x-l converter you have to balance that with
the practical fact that the shorter lens can quickly be changed by to a 300 mm
if the subject gets too close/too big, and unless you owna packhorse that follows you on a shoot---you won't take both lenses with you.