Originally posted by normhead My A-400 ƒ5.6 is 77mm. If it's 72mm it's 6.3.
I think that 77mm is the size of the filter thread. Did you measure the real diameter of the frontal element? I did with the Tammy 70-200/2.8 and Sigma 70-200/2.8. The frontal element was around 71-72mm, and of course, filter thread was 77mm.
72mm is the size of the clear opening on the level of the front element, because 400/5.6=71.428. Of course the frontal element can be bigger than 72, but cannot be much smaller. For f6.3, the frontal element must be 63.49mm, minimum. And generally, manufacturers didn't put oversize lenses, for obvious reason.
Of course, in reality, the frontal element can have 71mm, or somewhere around, some lenses branded as f2.8 are actually f2.9, and so on. but the theoretical figures are clear f = focal lens / diameter.