# Fourier Optics

I have been studying Fourier Optics and I have a basic conceptual question. I understand the mathematics of how to perform Fourier Transforms however the part of this topic I seem to have missed is why the action of a lens on light is the same as performing a Fourier Transform on the functional form of the input wave?
Other than having been told by my lecturers that the output of a lenses is the Fourier Transform of the input I have no idea why this happens?

I hope this question is clear. The whole concept is rather confusing to me.