Confusing Matrix4.Perspective

private void MakeCam()
        {
            aspect = (float)w / (float)h;
 
            if (orthoView)
            {
                GL.MatrixMode(MatrixMode.Projection);
                GL.LoadIdentity();
                GL.Ortho(-r * aspect, r * aspect, -r, r, -100, 100);
            }
 
            else
            {
                //Matrix4 perspective = Matrix4.Perspective(60, aspect, 1, 10000);
                //Matrix4 perspective = Matrix4.CreatePerspectiveFieldOfView(60, aspect, 1, 1000);
                Matrix4 perspective = Matrix4.CreatePerspectiveOffCenter(-r * aspect, r* aspect, -r, r, 1, 1000);
 
               Matrix4 lookat = Matrix4.LookAt(0, -20, 10, 0, 0, 0, 0, 0, 1);
 
                GL.MatrixMode(MatrixMode.Projection);
                GL.LoadIdentity();
                GL.MultMatrix(ref perspective);
 
                GL.MatrixMode(MatrixMode.Modelview);
                GL.LoadIdentity();
                GL.MultMatrix(ref lookat);
                //GL.LoadMatrix(ref lookat);
            }
        }