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There's nothing else this wide for use on a full-frame 35mm camera or digital such as the Canon 5D. It's so wide on a full-frame that you can almost see behind you (well not quite, but close to 180 degree field of view). It is not a fisheye, so straight lines stay straight. There is some minor vignetting at 12mm but less than the Canon 17-40 L-lens I had at 17mm. This lens is pushing the limits of physics so it is not as sharp as some lenses but it takes in so much more of the view and provides a crisp image all the way to the corners.
For use on a cropped sensor camera, such as the XT, 20D or 30D a better choice may be the Canon 10-22mm which is uber-sharp in the middle of the frame. The Canon, however, gives fuzzy images on the sides when wide open but sharpens up when stopped down to f8 or more. My third copy of the Sigma lens is less sharp than the Canon in the center of the frame but is sharper starting about halfway to the edge when both lenses are wide open. Near the edges of the frame the Canon gives a dark and mushy image when wide open at 12mm while the Sigma is only slightly less sharp than at the center truly amazing!
Unfortunately, Sigma lenses are seemingly sold "as-is" with wide sample variations. The autofocus died on my first copy of the lens after less than 30 pictures. The second copy had poor optics with soft and fuzzy pictures even when manually focused. The third copy is sharp at 12mm, even wide open, all the way to the edges, but only if manually focused. At 12mm the lens autofocuses too far away when an object is close and too close for distant objects. At 24mm it autofocuses right on but is softer than at 12mm. I've decided to keep it since it is easy to manually focus. The depth of focus is large at 12mm. For objects further than a few meters away when using 12mm just set the lens to infinity and everything from a few feet to infinity is in focus even at the edges of the frame and in the corners. The lens has a 4-year warranty so I may send it in to Sigma for calibration later.
Pros: (1) Unique ultra-wide angle perspective on a full-frame camera. (2) A good copy will be optically sharp all the way across a full frame, even wide open. Further, there's virtually no purple chromatic aberation near the edges of dark objects with bright backgrounds. (3) Nice build quality with good finish and large smooth focus ring. (4) Straight lines stay straight so no defishing needed (defishing uses interpolation which lowers resolution away from the center). (5) Lens comes with a case and has a small built-in hood (serves mainly to protect the front element). (6) 4-year USA warranty if purchased from an authorized dealer like Amazon or B&H. Otherwise you get a 1-year International Warranty.
Cons: (1) Wide sample variation getting a good copy takes luck or persistence while trying multiple copies. (2) The bulbous front element (this lens is nicknamed "Popeye") is exposed and no standard filter can be used for protection so be very careful. (3) Flares easily. The sun does not need to be in the picture just anywhere not behind you. This is a result of the ultra-wide design and not a fault of Sigma. (4) Photos sometimes come out too bright, giving the appearance of low contrast. Photoshop or other software easily fixes.
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