Are falcon lenses good and would they be good with a nikon d3000?

And also why is this so cheap?
http://cgi.ebay.com.au/500mm-P-Telephoto-Lens-Nikon-D5000-D3100-D3000-D300-/290467157896?pt=UK_CamerasPhoto_CameraAccessories_CameraLensesFilters_JN&hash=item43a1311788

That is a complete piece of garbage. It is cheap because you get what you pay for.

Indeed

I never even heard of falcon. That lens is probably a piece of garbage

They are super generic lenses that are what are called "preset" lenses in that you have to manually stop down the aperture to meter and take the actual shot.

They are very slow (f/8 wide open) and are attached to cameras using a T-mount adapter (not electrical connections to engage the light meters in the cameras

It is doubtful that you can even use the cameras light meter with these lenses.

Here is link to the real 500 mm lens made by Nikon

http://www.nikonusa.com/...ED-VR.html

You will notice that it is nearly $8600 USD

The design is from the 1960s so no R&D costs. IT can be made with low quality materials. The end result would be comparable to digiscoping with a cheap scope, in other words not very good.