concentric rings appear for me in my 5mm vixen eyepiece when i am just out of focus......... if you go tooo far, then you get the "cd rom" look as you have described.
now depending on seeing conditions, then these sharply defined rings can be blurred quite significantly.
Also i have found the quality of the eyepiece a big factor....the better the eyepiece ie vixens and even better the TV radians, then the better these rings.
http://legault.club.fr/collim.html
this is a good read.