Originally posted by rcolman Lets leave out current production lenses because, after all, all you need is a fat checkbook.
Putting old, soft glass in front of a K-1 is like booting a brand new 4 GHz workstation off a 3.5" floppy disk. It might be kind of fun, and you'll definitely end up with some interesting results, but if you care at all about performance, you're just wasting your time.
Old 35mm or longer fast f/2.8+ primes stopped down to f/5.6 will look OK, but the best glass you can get is the current lineup of lenses made by Pentax, Sigma, Tamron, and Samyang.
Old ultra-wide lenses look terrible and are essentially unusable at any f/stop, in my experience.
If you're interesting is dreamy, soft portraits, then open the lens wide-open and be amazed at the beautiful softness that results from 80s-era optical designs.
My opinion, however, has always been that if you're spending less money on glass than your camera body, you're probably doing it wrong. Since you mentioned you're looking for an ultra-wide, the 15-30/2.8 is the most usable UWA lens you can get for the K-1. If you're really pinching pennies, get the Samyang/Rokinon 14/2.8, though it's a manual-focus lens, relegating its use to landscapes, inanimate objects, and animals/people that are no longer living.