One of them are the fisheye lens, it is a wide-angle lens that takes in an extraordinarily wide and hemispherical photos.
Basically, the above mentioned lens is mainly developed for astronomy and known as the whole-sky lenses, then it quickly became famous in general photography.
It is best suited in full frame DSLR, like the Nikon D90.
Nikon D90 fisheye lens lets you capture some great landscapes and seascapes because you will be able to emphasize to curve of the Earth.
Its focal lengths depends on the cameras formats.
For the popular 35mm format, commonly focal lengths of the fisheye lenses are between 8mm and 10mm for circular lenses and 15-16mm for full frame lenses like the Nikon D90.
Often ultra-wide angle lenses agonize from some amount of distortion.
While this can be conveniently fix for moderately wide angles of views, rectilinear ultra-wide angle lenses with angles of view bigger than ninety degrees are hard to design.
Using a Nikon D90 fisheye lens commonly capture an image in a circular curves.
But it is true that fisheye lens is not a lens to use all the time, it is certainly not a good idea to keep attached to one's camera while walking around unless you are a photographer who cover all of your photos in fisheye and when sometimes the right picture does not come along, a fisheye lens can sometimes creates a stunning photos.
Popular use of fisheye lens is in the sport of skateboarding; photographers used fisheye lenses so they can get the camera as close as possible to their subject and still absorb a clear full view of their subject.
It is a one good lens that can be mounted on your digital cameras to acquire unique photographs.