Again, this may be more useful to me than to others, but here is a quick and easy way to make circular level sets in matlab:

[x,y,z]=cylinder([1 0], 1000);
contour(x,y,z)
axis equal
axis image
axis off

Told you it was simple. The 1000 parameter is what makes the circles more circular and less octogonal. By default it's set to 20, which isn't so pretty.