Tamron Announces the 100-400mm f/4.5-6.3 Di VC USD, Available November 16

[KevinP]

Re: Tamron Announces the 100-400mm f/5-6.3 Di VC USD, Available November 16

For this lens, 400 mm / 67 mm front element size yields f/6.3. This is irrespective of the imaging circle.
An f/5.6 lens will have a front element of 400 mm / 5.6 = 72 mm minimum front element size.
This is why telephoto lenses typically have the larger imaging circle covering 35 mm. There's no advantage to only cover APSC.

Do you have a good source where I can go read on these relationships? There's a non-intuitive thing going on if that's the whole answer. Is there a focal length range where lenses are f-stop limited, and a shorter focal range where they're coverage circle limited?

I see a lot on f-number ratio, but I don't see a lot on the actual aperture position. Is it a theoretical aperture size at the front element?

Most of the articles I find are piece-meal, with few or no illustrations to clarify what's happening.