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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); }