Originally posted by cali92rs Compared to the Sigma 18-35mm.
The Sigma 18-35 is not currently available for the K mount. When it does come out, it will be the only zoom lens that, at least at some apertures, out resolves the DA 20-40 (and probably not by a significant amount). The fact is, all standard zooms above kit level are plenty sharp. The fact that one is a bit sharper than another is irrelevent.
Originally posted by cali92rs when Pentax lenses don't perform well in tests, it doesnt matter because of "pixie dust".
"Pixie dust" is merely a metaphor to describe something people experience: namely, that tests don't determine how well lenses perform in the field. Case in point: the DA 10-17 may be the worst lens, in terms of test performance, of any lens by a reputable manufacturer currently in production. It's resolution is inferior to even to bottom of the barrel kit glass and I doubt you'll find another AF lens with worse purple fringing. But the actual images that the lens make, despite its optical weaknesses and its testing maldroitness, are stunning. Incredible colors, microcontrast, and flare control. Even though the lens counts for less than 2% of my total output, I've sold more images from that lens than any other piece of glass in my stable. The non-testable properties of a lens really can make a difference in the ultimate output. Thus all the loose talk about "pixie dust."
Originally posted by cali92rs FWIW, i don't think the Pentax lens is average. From the pictures I have seen, it is capable of taking great photos. My issue is that it seems like Pentax/Ricoh charge a premium just for being niche and different.
OEM glass tends to cost more money, sometimes a lot more money, than 3rd party glass. Simply compare Nikon or Canon lens prices to Sigma prices. You'll generally pay more for OEM. Is the extra price worth it? It depends on your priorities. If all you care about is aperture and resolution, then Sigma and Tamron glass constitute better value. But if there are other things that concern you, such as build quality, workability with camera, flare control, color rendition, than OEM glass might be preferable.
The DA 20-40 has four main advantages over the Sigma 18-35: size, build quality, flare control, and color rendition. In landscape photography, those advantages are significant. The Sigma obviously has advantages for hand-held low-light photography and for DOF control. But for landscape use, it's not a very compelling product. In any landscape images involving direct sunlight, the DA 20-40 would enjoy huge advantages.
The Pentax DA 20-40 is the lightest high-end WR standard zoom on the market. It's even lighter than the mirrorless offerings in its class, like the Olympus 12-40/2.8 and the Sony Zeiss 24-70/4.