According to that great compendium of knowledge, Wikipedia, a fisheye is "an ultra wide-angle lens that produces strong visual distortion intended to create a wide panoramic or hemispherical image."
Used subtly, the fisheye can bring a whole new perspective to familiar sights, as this selection of London landmarks demonstrates. The interiors of our great buildings — the Natural History (our favourite here!) and British museums, the Foreign Office — appear even more grand and cavernous. London underground's concave tunnels, meanwhile, become futuristic and claustrophobic given the fisheye treatment.
Finally, looking upwards brings a new look with this bit of photographical trickery too. The London Eye looks completely different while St Paul's becomes uncomfortably sandwiched between two brick slabs. Lots of fun all round.