Originally posted by shakereduced I still keep seeing people saying this is nothing more than a rebadge of the Tamron. It is not.
Here is the product page for this lens. Reading the text and looking at the lens design shown, this lens has an AD element (anomalous dispersion glass). The Tamron does not. There may also be a difference in the front aspherical element with the Tamron being hybrid and the Pentax being glass, but so far it's unclear from Ricoh's descriptions. Either way, it's at least an upgrade with the AD element over the Tamron.
Except the Tamron features two XR elements, the same thing as an AD (it's just marketing speak). The Tamron lens features weather sealing, quick shift, stabilization, advanced coatings and all the bells and whistles you could imagine, short of a focus hold button. It's an extremely advanced optic, and it would do you and other users well to read up on it instead of just imagining how it should be, in your minds.
On the other hand, the removal of the stabilization group probably afforded Pentax an opportunity to improve some aspects of the design by adding extra asphericals in there, which should be good news.
The part that really worries me is the lens sealing schematic shown by Ricoh. It looks like the lens has a low amount of seals, much less than we're used to. Which means, in turn, that they're just employing exactly the same dubious sealing of the original Tamron, which is more in line with Canikon lenses.