I'm doing a problem where we have plane monochromatic waves incident on finite double slits (each of width a, separation d), and the transmitted light is observed in the focal plane of a lens of focal length f. The position of the lens isn't specified. So I have worked out that the intensity distribution (ignoring the lens at the moment) in the Fraunhofer regime would be I(θ)=I(0)cos2(kdsinθ/2)sin2(kasinθ/2)/(kasinθ/2)2 However I'm finding it hard to understand how the lens fits into the problem exactly. If the lens was right after the screen, I would write sinθ≈tanθ=x/f where x is the position on the screen, and then rewrite my intensity in terms of x instead of θ - this works because the length between slits and screen is clearly f. However, if the lens was anywhere else, which it could well be, the distance between slits and screen wouldn't simply be f anymore. I don't see how the above could hold anymore, and so how would I rewrite the intensity in terms of f now?. So the problem is how does this work out when the lens is in any position Thanks for any help! Note, I would have this same issue if the aperture was simply two infinitesimal double slits with intensity pattern I(θ)=I(0)cos2(kdsinθ/2) so feel free to use this if necessary to simplify things.