An alternative to the FA50mm is the Voigtlander Nokton 58 mm f1.4. See review at Photozone.de {http://www.photozone.de/nikon--nikkor-aps-c-lens-tests/360-voigtlander-nokton-58mm-f14-sl-ii}. It is available with Pentax mount (eg Adorama.com and B&H). The Nokton 58mm is about the same price as the FA50mm and it is built like a tank, Very robust and excellent for outdoor. I took it during rainstorms and solid rain with my K-7 last month: an excellent low-light lens during the storm conditions, and I had no trouble despite the harsh weather.
The Nokton is not auto-focus but the aperture can be set to Auto (A position). It is an excellent performer, possibly in between the FA50mm and DA* with the price of the FA and the weather resistance of the DA*.
Hope that the suggestion can help. I definitely recommend the Nokton for outdoor: it is rugged and provides nice IQ.