I ordered this lens recently after seeing some positive comments on dpreview, backed up by a link to images, including one showing very good sharpness at f2.8. After ordering it I saw some comments here indicating that it is soft in the corners at f2.8. This was followed by Will Robb commenting that it is probably as good in the corners as anyone else's 20 mm. I would use this at f2.8 a lot. So I was concerned whether I should send it back.
I have access to a Sigma EX DG 20 f1.8, which I have been carrying when I travel. This weekend I tested both 20s against a mud brick wall (lots of detail) at several apertures, including f2.8. Today I processed the f2.8 images and looked at them onscreen at 100% (actual pixels) enlargement. At f2.8 the Pentax lens blows away the Sigma. It is noticeably sharper in the center, at the edges, and at the corners. And this is with the Pentax lens wide open while the Sigma is stopped down 1.3 stops. The FA 20 is a keeper and goes into my kit. The Sigma goes back to where I borrowed it. Incidentally, I shot the same subject with the DA 14. With both lenses at f2.8, the FA 20 is a far sharper lens. That said, both the Sigma and the DA 14 have given me satisfactory performance at f2.8. It's just that the FA 20 is much sharper. The FA 20 has the same optical formula as the A 20 f2.8, which was introduced in 1985. As far as I can tell, this lens has neither ED nor aspherical elements. Yet nearly 20 years later it continues to deliver top notch performance. And all of you thought that all I did was gripe. Joe
