... and I think Sigma would do well to spec a 135 mm prime at f/1.8
that would require filter size in the 8xmm size
Wouldn't 75mm suffice ?
Yes. In practice you'll end up with a 77 mm filter.
Canon's 135/2 has a filter size of 72 mm. Same as the 200/2.8
135 divided by 2 is only 67.5mm, with the barrel thickness, they end up with 72mm filter size
135 divided by 1.8 is already 75mm of opening, + barrel thickness, i don't think 77mm (+2mm) can do, thats diameter, not radius