i make planet rings out of a cylinder, with very high diameter settings (one larger than the other, say start=450 and end=500), almost no height (usually 0.01), and no start or end caps. i also use latitude divs=1, and longitude = something very high, maybe as much as 200 depending how big my planet is.
this method has the advantage that the rings are still an easily-editable primitive (useful if you change your mind about ring size later), rather than a harder-to-edit mesh or subdiv.
also, to zoom in or out when you're looking through the camera in scene mode, you can animate the camera's FOV property. bigger numbers = bigger viewing area.
- colclough