Is 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?
I didn't ask what the difference between an attachment and a lens are - I'm well aware of that. I'm not an idiot.. I was asking what the best kind of lens for my camera would be. And I'm also well aware of how much they'll run me.. I've done my research. My camera wasn't cheap, and I certainly wouldn't have spent that money if I had no intentions or money to expand my lens collection. I can't help but get the impression that you're trying to snub me or sound elitist.. nice try.. I appreciate you attempting to help, or be rude.. but try reading my question next time.

You need to understand the difference between a lens and a lens attachment.

The term 'fisheye' was originally applied to lenses of very short focal length (aka very wide angle lenses). All lenses project an image on the film (or sensor in digital cameras) that is circular. If the angle of the lens is extremely wide, that circle will fall entirely within the rectangular frame of the film or sensor, generating the circular image that is characterized as 'fish eye'. But there are lenses that are less extreme that are still sometimes referred to as 'fisheye' although that is probably not truly an appropriate term for them. Wide angle lenses tend to be rather expensive, and extreme wide angle lenses are very expensive. This is because they are expensive to make, and the market for them is very small. A true fisheye lens will likely cost several times what you paid for your Nikon D5100.

Attachments are gadgets that fit on the front of a lens and can include both color and effect filters, close-up lenses (aka 'diopters'), and gadgets that adjust the angle of view of a lens. There are a few aftermarket attachments that create an effect similar to a true fisheye lens. Lens attachments are much less expensive than lenses, but attachments that simulate the fisheye effect will introduce distortions that you would not have with a true fisheye lens. So the message is that you get what you pay for.

SIGMA 8mm F3.5 EX DG Circular Fisheye Lens

Tags: circular fisheye full frame, user manual, lense cap