Just ahead of time, no this is not related to homework or coursework in my case. I am a TA for an optics lab and need to know it so I can help the students in my class. I am trying to find an expression for the x-spacing for a grating imaged using the 4f method, which is a grating that is 1 focal length away from the first lens, which is then 1f away from the fourier plane, which is 1f away from the second lens which has a focal length equal to the first, which is finally 1f away from the image plane. Specifically, I am interested in knowing what the spacing is for the Fourier transform of a grating when it passes through a Fourier lens. I've heard it is somehow proportional to f*lambda/b, where b is the spacing between slits in the grating, f is the focal length of the lens and lambda is the wavelength of the laser, but I have not been able to find a source that solves such a problem. I would appreciate it if you could give me either an equation for the spacing or a more direct method for solving it. So far, the only methods I've found end with a k-space solution and don't try to translate the peaks you would see to actual displacements (x-coordinate).