What is a fish-eye lens?
FishEyeLens (picture from joe-ks.com)
Camera lenses have a so-called angle of view that shows the location similar to human vision, the limits within which the subject under focus and format can be photographed. The angle standard lens, which is roughly the angle of the human eye is about 50 °, whereas is the angle of a 15-mm fish-eye lens at 180 degrees (in 35-mm format diagonal) over the image. This means that almost everything from fish-eye lens, what is in front of the camera, captured and banished to a photo is taken, for example, the sky over the camera, the ground beneath the camera and far to the left and right, and the environment that we normally only see if you turned your head, because otherwise the human eye is not visible.
Since fisheye bring everything that is inside a picture angle of 180 degrees on a movie or an image sensor format 24 x 36 mm, it comes at the edges of the photo to severe distortions. All lines are straight out of the center of the photo is curved. The stronger the hyperfocal effect that everything is in the image reproduces sharp, the higher the items contribute in the middle. In contrast, objects that are located near the image edges, very badly distorted, which produced an overwhelming prospect. By the way Objective of this type were therefore referred to as fisheye, because they show the world just as they would see a fish, who sees through the surface through the top. The fisheye effect is produced by the refraction of light at the water surface.
When used fisheye should photographers because of the strength of this visual effect, care must be taken to have dictated the choice of picture is not from the lens, but from their own photographic or artistic tact. In the hands of an expert of this lens can open up a unique visual expression and also serve as an ultra wide angle, because the lines in the middle of the picture is not distorted.