Observing a Spherical ([[Light]]) Wave from $(x, y, z)$ while at $(x', y', z')$
The field in the observation plane, $E(x',y')$, a distance $z$ from the aperture plane is a sum of [[Spherical Wave]]s from every point with the aperature
$$
E(x',y') \propto \int \int \underbrace{t(x,y)}{\text{Aperature Fn}} \cdot \underbrace{E(x,y)}{\text{Electric Field}} \cdot \underbrace{\frac{\exp(ikr)}{r}}_{\text{Spherical Wave}}
dx\ dy
$$
$$
\text{where } r = \sqrt{ z^2 + (x' - x)^2 + (y' - y)^2 }
$$
See [[Fraunhofer Diffraction]], the simplified, approximate form