[skyclad666]

It is strange that no one mentions this lens as a super wide full frame option
LAOWA 15mm f/4 Wide Angle 1:1 Macro Lens | Venus Optics - Anhui ChangGeng Optical Technology Company Limited

It is super sharp with low aberration (except a bit of distortion).
Most of all, it is the only one to provide true immersive macro: be prepared to bring the object at 1mm of the front lens at 1:1 macro !
It accepts 77mm filters, contrary to some bulbous zooms. A polarizer can change your landscape!

It works marvelously well on the K-1 and super nicely on the K-5, with an additional shift function on APS-C. The lacking AF is acceptable at such short focals.

And all of this for $500 brand new.

[biz-engineer]

It works marvelously well on the K-1

Yes, that's an option. According to ephotozine , this lens deliver soft edge, even stopped down, that would be the case especially on FF, I've not seen real photos taken on full frame with this lens, so I don't know how relevant is the MTF test.