Hi, I own a Nikon D5200 and am looking to buy a fisheye lens. Can anyone advise me on the best mm lens for a cropped fisheye effect ie. rectangular fish eye not circular fish eye. I read that a 'full frame camera' will achieve this with a 15mm lens, or 10mm with a crop-sensor camera. Which is the Nikon D520?0 Also what brands would be recommended? I've heard Sigma is the cheapest but are they of a good quality? Cheers