Honestly a fantastic lens.
Most of the fringing is easily managed in light room, sharpness is far better than I expected. Probably won't be doing any prints of photos shot in this lens, but great for social media! Low price point is obviously a bonus. I use this lens for multipitch rock climbing. The fish eye wide angle works well on tight belays. The profile is tiny which is great for keeping your harness light. Also no lens cap to drop and never see again.
Would definitely pay full price again for this great little lens.

This lens is pretty good, its nice and sharp all the way through my pics, only thing i dont like is that its an f.8, i like quality so i hate cranking up the iso, to get good shutter speeds you gotta shoot in sunlight, even on a cloudy day the shutter speeds are much lower than id like with allot of my shots coming out not as perfectly clear as id like, but i cant minus a star for that, i bought it knowing it was a f.8, the focus lever is annoying i gotta say, i wish it was a ring around the lens, im always taking shot and notice the focus wasent perfectly in the place it should be, it has 3 focal points.

This lens is amazingly sharp through the center to at least 50% out. It produces a fish-eye distorted image with severe barrel distortion, but I found that using the distortion correction feature in Paint Shop Pro, I can correct the images perfectly!

This lens has a nice wide angle just as I expected. I was surprised at the sharpness of the photos taken with it, though. They are very sharp and compare well with the prime lenses that I owned and have owned. Amazing. It is manual focus and does not have image stabilization on a Lumix G7, but I can set the camera on iAuto and set the lens to focus at infinity and it becomes as easy as an iPhone to shoot with.

This cheap lens has quickly become one of my favorite m43 lenses. It's tiny, it's sharp, and the perspective matches the human field of view better than any wide-angle lens I've ever used.

So if you take a picture in a room with this lens, it represents how the room actually looked and felt in terms of space and distance, preserving the proportionality. Most ultra-wide lenses stretch things unnaturally at the edges, and most other fisheye lenses have a much more pronounced fisheye effect, bulging the center out. This lens seems to find the perfect balance.

In summary, this lens is a blast to use, and it's one of the best values I've ever found in lenses.

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The following is a detailed comparison I did on Pentaxforums between this "toy" lens and one of the best Ultra Wide Angle zoom lenses out there...the Sigma 8-16mm. Unfortunately, Amazon reviews do not post the full-sized images, as referenced in the review. My original review, which includes full-size pics, is titled "Just for fun: SIgma 8-16mm (K-mount) vs Olympus 9mm f8 fisheye body cap lens review"

I've decided to go to a dual Pentax DSLR / m43 system, but since I'm not made of money, I've had to make some difficult decisions regarding my DSLR lenses. I sadly sold my Sigma 8-16mm a few days ago, and I'm trying to decide between the Olympus 9-18mm or the Panasonic 7-14mm lens to replace it.

In the meantime, I found out about the Olympus 9mm fisheye "body cap lens", and figured that for the price it was worth a try to hold me over until I re-acquire a proper UWA lens. So how does an $88 body cap compare to the widest rectilinear lens available for APS-C cameras? For the price, better than I thought!

I've attached a couple pictures of the lens...er, body cap, mounted on my Panasonic GX7

The first two sample pictures are photos I took within minutes of unboxing the lens, as I was walking to meet my kids at the bus stop. The school bus picture had the top and bottom cropped, but it has not been resized; and the car picture is not cropped or resized. So if you click on the pictures, you can download the 100% picture to pixel peep it. [Full size pictures not available on Amazon reviews]

When I got home and loaded the picture onto my computer, the thing that amazed me is how sharp they were throughout most of the center part of the frame.

So the lens is usable, but how does it compare to the mighty Sigma 8-16mm? Well, in overall image quality, the Sigma definitely wins. The Sigma is sharp edge-to-edge, and has better contrast than the little fisheye. When shooting in RAW, you will probably want to add some zing to the fisheye pictures.

So if you're wanting to shoot an ultra-wide angle just for fun, and/or you won't be printing large images, then the Olympus lens can provide a lot of utility. But if you're doing critical work, then you'll definitely want to use a real lens.

The other thing I was curious about is the comparative angle of view of the two lenses. I was under the impression that the fisheye would be a little wider, since the 8-16mm is supposed to have a 121 degree angle of view on APS-C, while the fisheye supposedly has 140 degree angle of view. But in practice, it seemed like the lenses were very evenly matched. In fact, it seemed like the 8-16mm captured a bit more at the left and right edges, but I don't know if that's really a fair comparison, since the APS-C sensor is wider side-to-side than the 4/3 sensor.

The big difference is the way the two lenses capture the wide angle. The Olympus fisheye shows a lot of barrel distortion in the middle, and curves the edges of the frame, while the Sigma gives you straight edges, but gives you kind of a pinched view, with objects at the edge stretched and objects in the center of the frame shrunk.

I've never used a fisheye lens before, but my understanding is that the Olympus is not quite a true fisheye, since it does not provide a 180 angle of view. As far as I'm concerned, this mild-fisheye makes the lens easier to use in more situations since it is not too exaggerated. Depending on the scene, it may not even be obvious that the lens is a fisheye.

Here's a couple comparison pictures, and again, you can click for a full-size image. [You will need use the link above to view the Sigma/Olympus comparison pictures.]

I'm not sure that I prefer one approach to the other. The fisheye is cool with some scenes, but in other cases it just looks strange. And kind of the same thing with the 8-16mm.

One thing I do appreciate from the fisheye is that the proportions of a scene are better preserved. These next two images demonstrate that characteristic.

In the bedroom picture, the fisheye picture does a pretty good job of representing the actual size of the room. But in the Sigma version, the room looks larger than it is, since the bed is shrunk and the floorspace in front of the bed is stretch. And in the Sigma picture, the little mirror in the corner looks huge.

I think that either picture looks okay, but if these pictures were used for real estate purposes, the Sigma picture could be considered a bit misleading since it seems to add square footage to the room. (BTW, that is something I have noticed with some real estate pictures, and it always annoys me.)

For the second picture in my home theater area, both lenses were kind of a fail. Again, the Olympus picture does a good job of showing the relative size of the speakers to the screen, etc, but it just looks bizarre seeing the screen and speakers curved like that.

And the Sigma does a good job of keep the lines straight, but the screen looks tiny and the center speaker look tiny, and the L/R speakers look huge. And since they are canted a bit to aim towards the center of the seating area, the rectilinear lens makes them look oblong.

And for the sake of comparison, I've included a more normal shot showing pretty much how the home theater area appears in person. This picture was taken with my Olympus XZ-1 at 30mm equivalent:

So for everybody who has been dying for a comparison of those two lenses, there you go!

I'll finish up with a few pics comparing the physical dimensions of the Pentax K-30 & Sigma 8-16mm combo to the Panasonic GX7 & Olympus 9mm fisheye combo. (These pictures were courtesy of my old Olympus XZ-1 since that's the only other camera I have with a hotshoe!)

It still blows me away how little this thing is! There's been many times when I wanted to take my 8-16mm along, but I just couldn't bring myself bring add a 3rd heavy lens to my normal two-lens kit. Now I'll just be able to slip this little guy into a pocket or something, and forget I even have it with me until I want to use it.