when does a wideangle lens become wide enough to be classed as a fisheye?

i need a lens which will give me a good fisheye effect, but i'm a bit confused by all the different numbers like 0.57x, 0.66x, 0.3x etc.

would i be right in thinking that the smaller the number, the wider the angle of view, and the more the fisheye effect?
what lens would you recommend me for a good fisheye effect, im guessing between 0.3x - 0.5x?

any help very much appreciated, cheeers!
dunk :D

[Dubbing Mixer]

The difference between a wide and a real fisheye is this. A wide should keep verticals and horizontals straight. (Assuming they were in the first place!) This is difficult the wider you go and, basically, costs. A fisheye doesn't have to bother with such niceties and does not need the expensive and complex correction and should, at least in theory, be cheaper. E.g. I have a 17mm wide angle for my 35mm slr which cost a fortune. I could buy an 18mm fishey for a fraction of the price.

Bad wide angles produce curved verticals and horizontals to the point where they become 'fisheyes'

The lower the number (quoted this way) the wider the angle of view.

thanks for the speedy response!

ok, i have another stupid, and irrelevant, question, but i'm just curious...

is it possible to attach a lens to your camera which has a smaller thread than that of the camera's filter thread? do stepping down rings exist?

thanks

[Dubbing Mixer]

Yes, but there is a catch. You will most likely get vignetting at full wide angle settings and with the iris wide open. I.e. you'll get a circular shadow at the extremities of the image. Think looking through a round hole in a piece of cardboard held close to your eye.

Going the other way is a much better proposition. I.e bigger adaptor diameter than original lens.