AlanF said:
What I am getting at is that the DO technology not only reduces weight but it also reduces length. The 400mm DO is 35% shorter than the 400mm/2.8s from Canon and Sony. If Canon decides to make a 400mm f/4 DO it would be also ~35% shorter. It is perfectly reasonable to compare the advantages of different technologies even if the fruits of them have yet to be realised.
So far, we’ve seen DO lenses only as fast as f/4 (400mm prime, 70-300mm variable aperture zoom, and the prototype 600/4 DO from Canon, and Nikon’s 300/4 PF). Are f/2.8 or faster lenses amenable to diffractive optics? Possibly, there are patents for a 16-35/2.8 DO and a 70-200/2.8 DO. So far, nothing to suggest that a 400/2.8 DO will ever happen. Time will tell.
Incidentally, the patented 70-200/2.8 DO is only 10% shorter than the extant 70-200/2.8L IS lenses. Certainly a very different lens design, but extrapolating from one data point is not terribly reliable.
With DO lenses, the decrease in length seems relatively more significant than the difference in weight.