This just might be one of the most impressive pieces of glass ever produced. Nikon's Nikkor 6mm f/2.8 is an "extreme" wide-angle with a viewing angle of 220º that can cover an image area of 24x36mm. Produced in 1972, the rare lens is available at Grays of Westminster for the princely sum of $161,000. Link
Sigma announces a light-devouring 20mm wide-angle Art Series Lens
Sigma's acclaimed Art Series lenses gains a new wide-angle.