Extreme wide angle lenses.
Wide-angle lenses can be designed in two ways.
- They can be corrected to make all straight lines appear straight (rectilinear correction); but this type of correction introduces perspective distortion, i.e. objects at the edge of the frame appear stretched in comparison to objects in the middle.
- The alternative is to avoid rectilinear correction, in which case perspective distortion is minimized; but rectangles appear to bulge in the shape of a pin-cushion, and the only lines which remain straight are those which pass through the exact center of the field of view. The latter type of lens is called a 'fish-eye' lens.