I still think it's a decentered lens. I know many have praised the quality of their Rokinon/Samyang primes, but that has not been my experience. At the extremes, such as wide open at f/2.8, it does not have edge or corner sharpness and there is greater sample variation in quality from lens to lens. At f/5.6-f/8 they are fine, but not at max or min aperture.
The Pentax 14mm is more than double the price of Rokinon/Samyang for various reasons:
a) Pentax field of view: 90 degrees vs. RokSam 94 degrees
b) Pentax 12 elements in 11 groups vs. RokSam 14 elements in 10 groups
c) Pentax 420 grams vs. RokSam 530 grams
But beyond the tech numbers there is also quality control and overall IQ. I think your lens will be fine in the mid-aperture ranges, but noticeably problematic f/2.8-4.5 and probably f/13-22.
---------- Post added 10-15-18 at 04:10 PM ----------
Originally posted by ZombieArmy Also what do you mean by flat planar subject? I'll try to find something to do that with.
Like a flat brick wall where you can fill the frame from corner to corner.