And then, this is the Pentax FA20-35mm F4 Ultra Wide Zoom:
And then, compare the above two and then also against the Leica Summilux 21mm Prime below:
Via: http://www.dchome.net/forum.php?mod=viewthread&tid=1215807
(in Chinese)
It is not difficult to notice that, as the OP has pointed out, the optical designs of the three lenses are quite similar, especially for the Tri-Elmar and the FA20-35. So, who copied whose optical design? :-o
I have also checked the optical formula of the DA21 F3.2 Limited lens, it can be seen that the optical design could still be similar, although it is somehow simpilfied:
(Diagram Source: http://kmp.bdimitrov.de/lenses/primes/ultra-wide/DA21f3.2.html)
Nonetheless, as Pentaxians, we are of course glad to learn that we have Leica's optical formula embedded right inside our Pentax glass! So, Pentax could really be the poor men's Leica! ;-D