Camera: Pentax K10D Lenses I have: 18-55mm, 80-300 or something
Need: I want a wide angle lens. I want to be able to shoot landscapes with it, but also have it be able to shoot interior real estate nicely too...
So far im staring at these: http://www.pentaxwebstore.com/detail/PTX+21580 http://www.sigmaphoto.com/lenses/lenses_all_details.asp?id=3280&navigator=4
I'd love peoples insights, feedback, etc based on my wants there!
I would say neither...
The pentax is listed as a fisheye, which means it will distort the hell out of the Interiors - bad for real-estate listings. You need a rectilinear wide angle or zoom.
The Sigma 20mm is very good, especially for a sigma, but not wide enough on a crop sensor for interiors. (acts like a 35mm on film, nice for street stuff and landscapes, but not interiors).
I would say maybe the Sigma 12-24mm - that should be a more or less rectilinear zoom:http://www.sigmaphoto.com/lenses/lenses_all_details.asp?id=3236&navigator=1
I don't know who else makes a good lens in that range for Pentax mount... the Tokina 12-24 f4 is very good for the price, maybe the best aside from Nikon's - but I don't think they make anything other than Nikon/Canon mounts.
I'll poke around a bit.
Edit: Looks like Pentax makes a 12-24/f4 as well! That might be the way to go. http://www.pentaxwebstore.com/product_detail.asp?T1=PTX+21577
sweet r_d - the only question I would have is that I was putting A LOT of weight on the f-stop factor of that sigma down to 1.8. Those others start at 4...isnt that a major limiting factor for interior shooting?
You should be using a tripod for interiors anyway... 
The 20 1.8 is not sharp at 1.8... it's acceptable by 2.8, and sharp by 4... most of the f4 designs are pretty good at 4. (Plus you want the room sharp, at 1.8 with a 35mm FOV, there will be some depth of field related blurring).
And, 35mm equiv FOV won't do you much good for your need, that trumps the aperture's usefulness IMO.
There is a very good Pentax 14mm f2.8... but it's a bit pricier still.
As far as f4 being a limiting factor... when I shoot architecture professionally, I'm usually at f16-32 with the big cameras.
Now - if you want to capture pictures of your kid running around indoors, in low light, that's a different story, but for real-estate shots, just stabilize the camera, and bump your exposure times up.
wow, awesome feedback! im glad I asked the pro...
Ill check out those others, thanks!
ok, now ive confused myself into a corner. r_d, your inputs are highly valued on this list:
So they are all in the price range I am willing to pay...keeping in mind the things I want to do with the lens, Im at a loss. For instance, why would you buy the 15 over the 14 when the 14 is at f2.8 is it just that sexy TINY size of the 15, etc?
Keeping mind what you want to do, I think the 12-24 gives you the most flexibility.
It's the widest - which will help you with interiors. And being a zoom will help you with framing indoors. It also gives you the 20-24 range, which will be very useful for landscapes. Often the ultra-wides are a bit too wide (I find anyway).
As to why one would get the 15, vs. the 14.. it would come down to size vs. speed mostly - whichever is more important to a given individual. I would be curious to compare resolution and distortion charts on both before recommending one over the other... based just on raw specs, either one would work just fine for you. But both are less flexible (albeit, probably a touch sharper) than the 12-24, and since you stated multiple purposes in your needs, is why I still recommend it.
The 21mm suffers from the same primary problem as the sigma 20. Not wide enough for most interiors with your camera. If you were looking for a great walkaround lens for street and events/documentary stuff, it would be just fine (maybe a touch slow, but nothing major given its size), but that's not what you stated as your need.
Sweet man, thanks for the help! I'm picking the 12-24 up!
If you don't care about autofocus you can save a lot of money buying an older manual lens and get something much more swanky. An AF pentax pancake lens (DA 40/2.8) is about 500 USD, I got a manual one on eBay for 35 bucks plus shipping with an old Asahi Pentax body (in surprisingly good shape) thrown in. One of the best things about Pentax DSLRs (IMO) is their lens compatability.
Amusingly, I don't use the lens as I find it too small to comfortably adjust - the right grip sticks out too far. I guess with an autofocus that wouldn't be an issue. either way, i needed to replace my old Canon film camera anyhow.
Good call Ari, the only place that bites you a bit is on the wide angles. They just didn't make very many in ultra wide (wider than 21 or 18mm) - sure there were a few here and there, but on a film it starts to get ridiculously wide quickly. Once you have a crop sensor dSLR, to get true wide angle support (12mm-15mm) is needed. And those are hard to come by at bargain prices. (as a rule anyway).