Basically what you are saying is that it would look as a fisheye photo, is that it? At 12mm the photo will look like a circle? I donít suppose one can get an aspherical 12mm photo, is it possible?
No. Vignetting is a darkening of the image in the corners and edges because the image circle projected by the lens is not large enough for the format of the camera (or for reasons of an obstruction to the light at the edges, such as the rim of a filter). It is sometimes known as 'peripheral light fall-off', and is often accompanied by increased distortion and lens aberrations.
In other words, you have a crap image towards the edges.