Originally posted by pres589 I can't imagine that many people would buy the 12-24 instead of the 11-18 unless it's for cost reasons. 18mm is a perfect spot to change lenses to something else that covers about that point out to something longer. This isn't to say the 12-24 isn't without merit, it's just not a star lens, and Pentax usually does very nice things when they stick that label on the side.
The things that have to be balanced are IQ, cost, weight, size.
My Sigma 8-16 only goes with me when I have an actual use for it. It's too big to just lug around all over the place, takes too much space in the camera bag, and weighs to much. This would be bigger. I suspect, it wouldn't get a lot of use, because my Sigma 8-16 doesn't get a lot of use.
The 12-24 is 439 grams
My Sigma 8-16 is 555 grams.
No specs on the 11-18 yet but it will be a lot bigger, a lot heavier and a lot more money.
I did put a 15 ltd on my camera before I bought the Sigma, I liked the size and weight, but the images simply weren't better. On blind test with my wife selecting the best images it didn't match even the 10-17 fisheye in many respects. I'd like to have it for it's size and weight, but it's a lot of money to pay for an "easy to carry always with you" lens. That being said, withe the 21 ltd.. and 40 XS the 15 would be great companion so you'd have quality glass in a very small travel package.
Looking back, I probably should have bought it instead of the 21 ltd. I have other lenses that cover 21, like my 18-135, but portable 15mm is hard to come by. If you buy the 11-18, you still want your 15 on many occasions. It still does what it does very well.