the cheaper wide options

What are the options if you aren't willing to spend for the 15mm Pentax f4.0?

Looking for something similar focal length but cheaper.

Digital King DSW Pro 0.7x Wide Angle Lens Pentax 52mm £69.99

Prime or Zoom?

Pentax DA 16-45mm f4 is very good and you should be able to pick one up for under £200 used.

Samsung D-Xenon 16-45mm f4 ED brand new £180.00
http://www.parkcameras.com/6815/Samsung-D-Xenon-16-45mm-f4-ED.html

Prime is fine

rosesred

same apertures and filed of view only 3 degrees less
Angle of view 83 - 35 o 86 o

http://www.lenstip.com/porownaj.php?co=obiektyw&ile=2&add0=318&add1=822

I wonder if the optical quality of the 15mm would be massively noticeable over the 16-45 @ 16mm

Usage would be star trials and infrequent urban stuff

16-45 is a superb lens. It is sharper than many of my primes at most focal lengths and also has surprisingly good bokeh for a zoom lens. It is resident on my K-5 almost 100% of the time.

Regards
David

I can recommend the Tamron 17-50

link

If you'd consider a zoom there's the Sigma 10-20mm (£329 at SRS) or 8-16mm (£399 at SRS.) The 10-20's often come up second hand, recently around £230 through the forum classified ads here. 8-16's are scarcer as it is a fairly new lens but still good value even brand new. An excellent alternative if you want Pentax is the 12-24mm. But these are all zooms, so for a prime the Pentax 14mm f/2.8 DA ED (IF) is a little bit cheaper than the 15mm f4 and optically good but much larger and heavier.

John

lots of options

the Samyang 8mm FE looks worth investigating also