Are there any lenses for Nikon that work as both a circular and diagonal fisheye lens?

If not, what would the best circular fisheye lens for the Nikon D5100 be?

The least expensive would be the Opteka 6.5 mm lens, but to get diagonal coverage, you will need a lens like the Nikkor 10.5 mm

Sigma makes both a circular and diagonal fisheye lens, but no combo lens. The circular lens would certainly be better than the Opteka.

For the diagonal lens, the Nikon 10.5mm is the best, but you can't autofocus it with your camera as it is an AF lens, not an AF-S lens. This is the only AF DX lens Nikon makes for some reason. Perhaps there was not room enough in the lens for a focus motor?

Also, some cameras and post processing software is capable of mimicking circular fisheye lenses, so look to see if you may already have that capability.

I have the Nikon 10.5mm (diagonal) fisheye, and you can make some very nice artistic impressions with it. Fact is, along the major horizontal and vertical axis, the lens is not distorted, but rather a super-wide angle.

You can get some interesting effects with it. On the other hand, a circular fisheye is just like looking down the center of a tube, and I think you would soon tire of it.

I would go with the more creative diagonal fisheye myself.

I would like to add to AWBoater answer you really don't need a autofocus for a fisheye lens