There are two flavors of fisheyes - pickled and fresh.
(Sir - wrong topic)
what do you mean this isn't a cooking writeup?
(This is a photography writeup)
You mean I ate those... ewww...
Ok, there are two flavors of fisheyes - full frame and circular frame.
The essence of a fisheye lens is to get an extremely wide angle of view - typically 180 degrees or more. With most lenses (including those in our eyes), straight lines are rendered as straight lines. This is known as rectalinear. Even with wide angle lenses, this is corrected for (or nearly corrected for). Small curvature at the edges is forgiven by the brain or just overlooked altogether. The fisheye lenses are deliberately not corrected for this - and thus the difference between a fisheye and a super wide-angle lens (which do render straight lines as straight lines).
The classic fisheye lens is the circular frame and provides "edge to edge" 180 degree angle of view. These lenses only expose a portion of the film but also capture a full hemisphere (180 degrees by 360 degrees).
Sometimes, these lenses are even wider and capture up to 220 degrees from edge to edge (I've heard of one capturing 270 - yes, thats right 270 degrees - you can be behind the camera and still be in the photograph).
The other 'flavor' of fisheye lens is a full frame fisheye. These lenses do not have the same field of view from edge to edge as does the circular frame fisheye and instead have a 180 degree field of view from corner to corner. These are still fisheye lenses despite the less classical viewing - they still render straight lines as curves.
There is a big difference between the full frame fisheye and the circular frame fisheye - and that is price. The Nikon 16mm f/2.8 fisheye lens (full frame) runs about $800 new. Meanwhile, the 6mm f/2.8 circular frame fisheye runs a tad more - on the order of $14,000. The full frame fisheye is often more useful unless you specially want the circular frame for effect. The full frame allows you to capture almost a complete hemisphere (though not quite) and is useful for concerts, landscapes, super-wide group photographs - you name it.
Ok, so you want to take a photograph with a fisheye lens now. What
should you keep in mind?
- Lens flare. Lets face, it - if you are outside it is almost impossible not to get the Sun in the frame. This will cause some flare. In most every case, these lenses try to reduce flare but it is very difficult. You can't stick your hand there to block it - your hand will be in the frame. Which leads us to...
- Strays. A fisheye lens will capture at least 180 degrees in its field of view. With such a wide angle it is quite easy to get a tripod leg, finger, or something else in the photograph. This is especially the case with fisheye converters for digital cameras where you aren't looking through the viewfinder and rather at the LCD panel.
- Straight lines. It is possible to get a straight line straight - run it through the center of the image. This is most often done with the horizon. Looking down just a tad will bow the horizon to the bottom, and looking up will bow it up. Precision is necessary to get it through the middle. This also is true of lines that run vertical (say, a lamp post).
- Filters. It is impossible to put a filter on the front - it just doesn't work. Polarization is out of the question. The filters are often special ones that screw into the back of the lens - bayonet mount.
- Depth of field. Ok - lets just say "its in focus". I'm not kidding. Focusing at 10 feet away, everything from 5 feet to infinity is in focus at f/2.8 - and if you go to f/16...
- Have fun! The fisheye lens is one that has 'fun' written all over it. Goofy photographs of people with the face all distorted to photographs looking up under a tree - this is a lens for having fun.