By Scott Gietler
The properties of light and water, and the refractive index of water leads to an interesting effect known as Snell's window. You will see a large circle of light, too large for most lenses, if you look up on a sunny day. Through this circle you can see whatever is on the surface, like the clouds, especially if it is calm out. This window can be used for interesting compositions, especially with a diver or large subject in the middle.
My friend inside of snell's window, Catalina island, mid-day in 15ft of water. Nikon D300 camera, Tokina 10-17mm fisheye at 10mm, F16, 1/320th. Even with a 10mm fisheye, I can't get all of Snell's window in the photo. You have to lay back and shoot straight up to get the window in the photo.
Lens Choice for Snell's window
I recommend a wide fisheye lens like a 10mm fisheye lens. Snell's window is very wide and even a 10mm fisheye (on a cropped-sensor dSLR camera) can't get it all in! A circular 8mm fisheye lens can, however.
Great photo of snell's window. It's always nice to have something framed in the window, like a diver and a manta, if they both happen to be around. Photo by Bonnie pelnar in Socorro islands, Mexico. D200, F6.3, 1/125th, ISO 100, Nikon 10.5mm fisheye. Manta and diver were lit by strobes.
Snell's window by itself. Here you can see the clouds in the sky. F11, 1/125th, 10.5mm fisheye lens. Photo by Bonnie Pelnar, very close to the surface.
Here is some more info on Snell’s window: