Ultra Wide Angle Photography

From a design perspective, a wide angle lens is one that projects a substantially larger image circle than would be typical for a standard design lens of the same focal length; this enables either large tilt & shift movements with a view camera, or lenses with wide fields of view.
More informally, in photography and cinematography, a wide-angle lens refers to a lens whose focal length is substantially shorter than the focal length of a normal lens for the image size produced by the camera, whether this is dictated by the dimensions of the image frame at the film plane for film cameras (film format) or dimensions of the photosensor for digital cameras.
By convention, in still photography, the normal lens for a particular format has a focal length approximately equal to the length of the diagonal of the image frame or digital photosensor. In cinematography, a somewhat longer lens is considered "normal".
There is an easy formula for calculating the angle of view for any lens that produces a rectilinear image. In addition to giving a wider angle of view, the image produced by a wide-angle lens is more susceptible to perspective distortion than that produced by a normal lens, because they tend to be used much closer to the subject.