Over the years I have owned several fish-eye lenses. These lenses tend to create an abstract view by making no effort at all to correct for the ultra-wide angle distortion curve. Where an ultra-wide rectilinear lens had engineers go to great extent to correct this distortion, fish-eye lenses do the opposite. This need to correct distortion is one of the things that makes full frame lenses wider than 20mm either expensive or crappy.
Manufacturers use exotic glass and materials in these ultra-wides to correct the distortion and that leads to high prices. You really can’t get away with a cheap ultra-wide. But fish-eye lenses are a different animal altogether. These lenses do not require any exotic materials. It is possible to get decent results with a fish-eye lens that is modestly priced. The glass elements are still more expensive than standard lenses between 28mm-100mm full frame. Primarily the difference between a cheap fish-eye and a pro-grade fish-eye will be in absolute sharpness and lens flare.
There are three basic types of fish-eye lenses. There are fish-eye adapters that attach in front of a regular lens. These are usually cheap quality but can still be fun. There are a few high quality fish-eye adapters. Don’t be fooled by the super-cheapies, a 0.25x or less is required to have a “real” fish-eye effect with a modest prime lens behind it. Next is a full-frame fish eye. Don’t confuse the use of the term full-frame as meaning full-frame digital cameras. Full frame fish-eyes generally have 180 degrees of coverage from the corners across the diagonal of the image. On a 35mm equivalent camera theses lenses are generally 15-16mm focal length. The third type is the circular fish-eye. This lens on a 35mm equivalent body will have a focal length of 7mm-8mm. This projects a full 24mm circle with a 180 degree view inside the frame. These lenses will waste a lot of “space” since they will not use the whole frame. There are a few exotics such as the ultimate fish-eye, the Nikon 6mm f/2.8 that is a circular fish-eye. The lens that covers 220 degrees in all directions. This lens sees behind itself! This lens was created way back in 1970 and one of them recently was offered for sale at $160,000 and was quickly snatched up by a collector. Yes there is a lens that you could consider that might put you in the dilemma of having to choose between and Audi R8 or a lens 🙂

Canon 7.5mm f/5.6 fish-eye, scanned from 35mm Ektachrome slide, Safari West, Sonoma County, CA circa 1993
I have owned two circular 180 degree fish-eyes, two 180 degree full frame fish-eyes and a few fish-eye adapters. They all have their place. Back in the eighties I had two Sigma fish-eye lenses. I had the circular 8mm f/4.0 and the full frame 16mm f/2.8. The 8mm was a pretty good lens, it was a full stop faster than the Canon 7.5mm f/5.6 but I later owned that lens also and the Canon was a sharper lens. The Sigma 16mm turned out to be a great lens, I never bothered to buy the FD 15mm f/2.8 from Canon. After switching to auto-focus in the 90s I bought the Canon EF 15mm f/2.8 and that was a great lens as well. Today I shoot with a Nikon fish-eye adapter that actually has more than 180 degrees of coverage depending on what lens it is in front of. It is the FC-E9. It is designed to work with video and smaller sensor cameras so it has some weird focus issues on full frame sensor cameras. I am currently using it with an old SP 35-80mm Tamron zoom. That lens has super close focusing ability which seems to help overcome the focal length issues and infinity focus.

Montgomery Street, San Francisco, CA with Sigma 16mm f2.8 fish-eye, circa 1985 scanned from Kodachrome 64 slide
Honestly you can find that old Sigma 16mm 2.8 pretty cheap at camera shows or on EBAY. They run about $150-$200. That was a great lens and you can find it in most manual focus 35mm camera mounts. These lenses really need to be shot on a full frame 35mm equivalent camera like a Sony A7 or something because without the full 180 degree view it will just look like a cheap wide-angle. That said the circular fish eye lenses can be effectively used on micro 4/3 or APS-C bodies as they will look like a full frame fish-eye on those cameras. Sigma still makes both a circular and full frame fish-eye lens for modern cameras and they make them for crop sensor cameras as well. They tend to run in the $300-$800 price range with the circular models being at the higher end.
I traveled all over the United States back in the 80s and 90s, carrying both the circular and full frame fish-eye lenses. I captured many of America’s biggest cities with that awesome abstract viewpoint only afforded by a fish-eye lens. There are many fun things one can do with a fish-eye lens. It doesn’t have to be expensive, even a super cheapy like the one I have for sale on EBAY at $15 can be fun.
I shot a few images through my Nikon FC-E9 0.2x adapter and frankly I don’t have the right lens for this product but I still got some cool images. It was designed for Nikon Coolpix cameras not DSLRs. With the “longer” focal lengths in larger sensor cameras this lens has some strange focus issues that require a close focusing lens to get infinity sharp. I know that seems weird, but it’s true none-the-less. It works well with my EOS-M and the 22mm pancake, but I am afraid to mount it to the lens since it is so heavy, I just held it in front of the lens for the test. In the shot with the red flower some of the leaves are laying flat up against the glass, the flower is less than 1/2 an inch (12mm) from the glass! In the image where I am looking straight up a couple of stray raindrops landed on the lens and they are almost sharp! The Nikon FC-E9 is big and bulky and HEAVY, so I won’t mount it to an AF lens or a lens with plastic filter threads. It is sharp and crisp however and seems to be flare free.
The bottom line remains that if you want to horse around and have some fun, a cheap fish-eye adapter can provide that. If you want sharp crisp images you should look at some of the real lenses offered for most system cameras.
Here are some fun fish-eye shots.

Nikon FC-E9 mounted to Tamron SP 35-80mm focus set at about 1.5 feet to get infinity. Rain drops on the glass are near sharp!