Originally Posted by ricardobeat 
The four thirds system was designed specifically for digital sensors. It's optical design allows the light to reach the sensor more evenly, at more straight angles.
Yes, I know the whole 4:3 story, being an OM user for years. Its one reason I chose to go with Pentax, and not Oly for digital.
You can give any marketing excuse you want for the amazing 4:3 system, but there are zooms and longer lenses for these APS-C cameras that don't exhibit CA. Therefore it is doable.
I just hope Pentax made sure to look closely in designing this lens for this one to exhibit this lack of CA or PF.
Especially since my sample of the Tamron is better than the one you saw, shows that partly it is build tolerances. Sigma, which you invested far more hundreds into after dismissing the $149 Tamron is known for producing many bad samples of otherwise good lenses.
Tight build quality tolerances like in Oly lenses, and corrected optical design is needed for the Pentax lineup as well. Especially since varying design of longer lenses yield different levels of CA with no CA examples in existence. And, they don't have to be 2kg.