You need to hook Auto Mip-mapping in render settings.
This makes it possible to parse a bias to the blur float value.

shader TextureDOF
(
point Po = P,
float strength = 0.022,
float Time = 0,
string fileName = "",
output color Out = 0,
)
{
point p = Po;
float focus = sin(Time)*.35+.5;
float blur = strength*sqrt(abs(p[1] - focus));
Out = texture(fileName, p[0],1-p[1], "blur", blur);
}