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answered 12 years ago
You might be able to get implicit_plot3d to do it. Maybe something like the following would work for you.
implicit_plot3d
var('x,y,z') f(x,y,z)=x^2+y^2+z^2/4 p=Graphics() for k in [1,2,3,4]: p+=implicit_plot3d(f(x,y,z)==k,(x,-2,2),(y,-2,2),(z,-5,5),opacity=0.4) p