Because they have to bend light from such extreme angles, ultra-wide-angle zooms are complex devices with a lot of elements. Also, exotic specialty glass, like the Sigma FLD element here, is often used to maintain sharpness and minimize aberrations.
Standard depth-of-field tables and the depth-of-field scales on the few lenses that have them are based on a certain-sized circle of confusion, which, in turn, is based on the eye's ability to detect unsharpness in a given-sized print at a given viewing distance. If you produce a bigger print, or view it at a closer distance than this standard or view the image at 100% on-screen, there will be less depth of field than indicated.
With a given camera, the controls you have over depth of field are focal length, shooting distance and aperture. Depth of field increases as aperture size decreases (i.e., as you stop a lens down). Depth of field increases as camera-to-subject distance increases, and it increases as focal length decreases. Ultrawides have very short focal lengths, which increases depth of field, but they also generally are used at very close range, which, in turn, decreases depth of field.
Diffraction. When light rays travel through an aperture, they tend to spread a bit. The smaller the aperture, the greater the rays spread. This is diffraction, and it reduces image sharpness. Because the ƒ number is the ratio between the diameter of the aperture and the focal length of the lens (e.g., a 100mm lens set to ƒ/4 has an effective aperture diameter of 25mm), wide-angle lenses have smaller aperture diameters at a given ƒ-stop setting than longer ones, and thus tend to exhibit more diffraction effects than longer lenses. So, it's generally best not to stop down past ƒ/8 or ƒ/11 with a super-wide-zoom lens unless you really need the extra depth of field. Shooting with an ultra-wide-angle lens at ƒ/22 will provide lots of depth of field, but overall image sharpness will be reduced due to diffraction—ƒ/22 on a 12mm lens has an aperture diameter of just half a millimeter.
Ultrawides Vs. Fisheyes
The widest ultrawide focal lengths (12-16mm for full-frame, 8-12mm for APS-C) can be found in full-frame fisheye lenses for these formats, as well. What's the difference?
Without going into things like central projection vs. stereographic projection vs. equisolid angle projection, the differences from a practical standpoint are two:
1Fisheye lenses produce a 180° angle of view (measured diagonally), while ultrawides produce angles of view in the 99°-122° range.
2Ultra-wide-angle lenses, theoretically, render straight lines as straight lines, wherever they are in the frame, while fisheyes curve any straight lines that don't pass right through the center of the image. In practice, ultrawides do curve straight lines near the edges of the frame a bit—less well corrected lenses more so than others—but fisheyes are designed to do it. Because of their tendency to curve straight lines, fisheye lenses are special-effects lenses, not suitable for "normal" photography. A full-frame fisheye landscape will call attention to the lens used for the shot, while a landscape shot properly with an ultrawide lens won't. This doesn't mean you can't use fisheye lenses to do landscapes; just be aware that the effect can easily overpower the subject/scene.
There also are circular fisheye lenses, which produce circular images rather than frame-filling ones. Circular fisheyes have even shorter focal lengths—6-10mm for full-frame cameras, 4.5mm for APS-C. Canon's new EF 8-15mm ƒ/4L Fisheye USM lens produces a circular image at 8mm on full-frame cameras and fills the frame at 15mm; on APS-C cameras, it fills the frame at 10mm.
A fisheye used for a landscape will severely distort vertical and horizontal lines. It's an effect that should be used sparingly for these subjects. Underwater photographers, however, routinely and extensively use fisheye lenses.