Here we assumed the lens is a collection of point masses M i at angular coordinates θ θ → → i and distances z = D i . Use sinh − − 1 ⁡ ⁡ 1 / x = ln ⁡ ⁡ ( 1 / x + 1 / x 2 + 1 ) ≈ ≈ − − ln ⁡ ⁡ ( x / 2 ) for very small x we find