I have a Tamron 16-28 mm lens. Want to buy a wider one. Searched what is good and found out Canon 8-15 mm f/4 and 11-24 mm f/4. Both are L lenses. Both seems good. Both full frame lenses. 8-15 mm lens is wider fisheye but almost half price of 11-24 mm. I am very confused after reading the reviews of Ken Rockwell about both lenses. Rockwell says : " it's the world's widest ultrawide lens" for 11-24 mm. Isn't 8-15 mm wider than 11-24 mm? I see 8-15 mm is fisheye and 11-24 mm is ultrawide... why 11-24 is much more expensive?