What are wide-angle lens?

Hi,
Jeremy asked so I post. His question
What kind of lens are wide-angle lens?
I going to explain crop factor. I will try to put it as simple as possible.
For cameras that you see in stalls now.
Crop Factor
Nikon DX: 1.5x
Nikon Full Frame: 1x

Canon DX: 1.6x
Canon Full Frame: 1x

So, what kind of lens fall under wide-angle?
Every lens have a focal length e.g. Nikon and Canon kit lens are 18-55
So, you take the focal length multiply by the crop factor.
If the number falls below 28mm, it should be a wide-angle lens.

Next, there are also ultra wide-angle lens. I'm not sure for the range for ultra wide-angle lens but an example is the Nikon DX 12mm-24mm. When mounted on a DX camera say the D90, u multiply 12 and 1.5. Do some simple calculation and you get 18mm. Which is very wide!
To go even wider...
Mount the Nikon's ultra-rare 13mm f/5.6 manual focus lens on an FX camera. That's wide! If you want to get the same focal length on DX, it would have to be a 8.6 lens!

Just a side note, Nikon have made only about a few hundred of this 13mm f/5.6 over the years. If you can get one now under $10000, you have just picked up a super deal. The average price is about $30000 +... Too expensive for me.
At least now.

Tyy