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The Pentax SMC DA 15mm f/4.0 Limited Lens is my number one choice for landscape work. It produces very little distortion and controls chromatic aberrations quite well for an ultra wide lens. The lens is very well built and while expensive (currently $550)...it is well worth it's price tag. It produces very sharp images when the aperture is stopped down at f/8.0 or f/11.0. This lens produces an extreme amount of contrast and also produces the best sunburst I've seen a lens produce while also reducing flare. You can point this lens straight at the sun and come away with a useable image. See here (http://scott-mcguire.artistwebsites.com/blogs/pentax-smc-da-15mm-limited-example.html)
While I primarily use this lens for landscape work, it also works well for real estate and architecture work as well. It also works surprisingly well when used as a close up composition. You can see an example here: http://scott-mcguire.artistwebsites.com/featured/gold-and-silver-ore-crusher-scott-mcguire.html