The Fourier transform of f(x) is given by:
This is the most common point of confusion.
A poor solution merely writes: [ U(x,y) \propto \textsinc\left(\fraca x\lambda z\right) \textsinc\left(\fracb y\lambda z\right) ] and concludes.
$$ F(f_x) = \int_-a/2^a/2 (1) e^-j 2\pi f_x x dx $$