Summary: I picked this one out for my Sony Alpha 77 from all the other options, because this one has autofocus. I take a lot of self portraits and I needed autofocus for that purpose. It is exactly what I expected, works well in low light, sufficiently sharp, and fun to use. I just think that price tag is a bit high, when you can get a manual focus one for $250.
Summary: A nice, ultra wide angle lens. The HSM focus drive is fast and quiet. One great added feature is the adapter for the lens cap. It has 72mm threads, so unlike most ultra wide angle lenses this lens can be fitted with a protective filter, which is very important for valuable lens optics. Granted the added filter causes some vignetting, but with the circular fisheye nature of this lens the small added amount of vignetting is not really an issue.
Summary: Perhaps I got a bad lens, but this lens is far too soft for my purposes. Remember, 180 degrees of vision is squeezed into a circle within the rectangular canvas of your camera's sensor (hence circular fisheye). I intended to use this for spherical panoramas for realtime background images, but the clarity is just too low on my D5100. For that purpose, stay away from fisheyes and go wide angle so details are preserved.
Summary: This product is great except cost. If you don't mind spending around $900 then this lense is for you!!!! If you are budget minded you could get a manual lense far cheaper that produces the same great pics. The only reason for 4 stars is cost everything else is wonderful.
Excerpt: Focusing can be done on objects that are only mm away. This leads to my biggest concern when using this lens: the danger of hitting the object that i'm focusing on, resulting in a scratched front optic.<br /><br />So far, I have had a lot of fun photographing every day items resulting in unusual results. <br />The main draw back to this lens, in my opinion, is that it is very pricey for what could be considered a "novelty" lens.
Pros: A really wide angle lens, Easy To Use
Cons: Can't use a filter, Easy to bump optics, See a barrel in photos
Excerpt: Image quality is superb, the contrast pleasantly surprised me. Even at f2.8 I do not see vignetting worth noting in the area of view - however, most of your image is of the barrel.<br /><br />I was surprised at how small the image circle, probably only 1/3 of the view finder is filled with the circle of view. I may yet return this because by the time you crop the image to get a print there isn't much resolution left to play with.
Pros: Clear Optics, Easy To Use, Firm Mount, Linear distortion, Strong Construction
Summary: Combined with Nodal Ninja rotating tripod heads, I'm using this lens to produce equirectangular 360deg x 180deg panoramas. Six shots in 60deg increments work perfectly when stitched with Hugin or AutoPano. Limited depth of field is a problem --- I find that typically I have to operate the lens at f/11 or f/16 to get decent full-frame-in-focus results, which necessitates shutter times of > 0.5s indoors, so it's only really useful for static scenes.