Originally posted by MarkJerling No idea. But, both the 50mm F1:1.4 and the 50mm F1:1.7 have the same filter size of 49mm. So I'm not sure the "direct" calculation holds true.
I probably didn’t explain clearly. The premise of the comment in the OP was simply that a 135/f2.5 lens with filter thread of 52mm couldn’t physically fit it’s theoretical maximum aperture (54mm). The filter thread bit is slightly misleading, what they’re really getting at is the front element diameter (which is always smaller than the filter thread).
By this math, a 50/f1.4 has a max aperture of 35.7mm and a 50/f1.7 29.4mm, so a 49mm filter would be fine in both cases.