It is actually:  yPos = ((xPos*xPos)/((xPos*xPos)-1)), I just
said -x*x/x*x-1 to keep it simple.  As for the focal blur, I didn't use
anything other than the basic shape, and a simple texture (and I think it
was the texture that caused the blur, I noticed that too).  The reason I
mentioned calculus was because this was an example in my calculus text book.
The whole scene, if you are interested was:

camera {
  location  <1,1,-5>
  look_at   <0,-1.5,0>
}

light_source { <4,1,-6> rgb <1,1,1> }
light_source { <0,-1,0> rgb <.5,.2,.2> }

#declare xPos = 0;

#while (xPos < 16)

 #declare yPos = ((xPos*xPos)/((xPos*xPos)-1));
 #declare rot = 0;

 #while (rot < 360)
  sphere {
   <xPos,-yPos,0>, .1
   texture {
    pigment {
     bozo
     turbulence 1
     colour_map {
      [0.0 rgb <.9,0,0>]
      [0.5 rgb <.7,0,0>]
      [1.0 rgb <.9,0,0>]
     }
    }
    finish {
     specular .2
     roughness .2
    }
    scale .06
   }
   rotate <0,rot,0>
  }
  #declare rot = rot + 10;
 #end

 #declare xPos = xPos + .1;
#end

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