In all consulted literature, the transfer function of the free space is given as follows: $$\exp(-i k_z d) = \exp(-i2 \pi d \sqrt{1/\lambda^2 -\nu_x^2-\nu_y^2})$$
When referring to this source, they derive the transfer function from the following equation: $$H(\nu_x,\nu_y) = \frac{f_{in}(x,y)}{f_{out}(x,y)}$$
I'm wondering why they do it this way (note: I've already seen this in other sources). I thought the transfer function is defined in the frequency domain rather than in the time domain (frequency and space for the fourier optics respectively).