Originally posted by Adam The lens is probably no better than a consumer zoom would be these days at 18mm- this has been my experience with the k15mm.
Allow me to disagree.
In a (fairly well controlled) comparison of K15/3.5 and Sigma 10-20 set at 15mm, I found
- that center performance was similar in terms of sharpness, with visible differences mainly due to white balance control;
- that K15 outperformed Sigma 10-20 in terms of off-center sharpness and distortion control.
A similar comparison between K18/3.5, Tamron 17-50 at 18mm and Pentax 18-55 at 18mm revealed K18/3.5 as a clear winner (except for the presence of chromatic aberration, which - as often with lenses from the manual era - is stronger in K18/3.5).
In addition, let me quote from my comments on K20/4.0 in the lens review section: "I have compared the K20/4.0 on digital with the DA21 and the Tamron 17-50. My conclusion is that K20 outperforms the other two, in terms of detail rendered and in terms of liveliness of colors. It is not as superbly sharp as K28/3.5 or K35/3.5, but it's better than the widely acclaimed DA21 or a very good zoom like the Tamron."
I imagine there will be other voices to make themselves heard, but my experience concerning K-series lenses in comparison to modern zooms (and even some primes) is consistently in favor of the K's.