Now suppose we integrate over a screen far away in the xy plane, which is small enough for the small-angle approximations to be appropriate, but large enough that we can integrate the intensity over − − ∞ ∞ to ∞ ∞ in x and y with negligible error. In optics, this is equivalent to summing over many fringes of the diffraction pattern. By the method of stationary phase, we can approximate f ( θ θ ) = f ( 0 ) in the below integral. We obtain