An optical lense is essentially transforming the image into fourier space and back again. The radius of the lense (approx) is a limit to the spacial-frequencies, i.e. some frequencies are clipped off. In the image plane, the signal has been backtransformed - however, some frequencies were lost due to the aperture.

I.e. the fourier transform explains, why apertures limit the resolution of optical devices. It is not only absolute brightness, but fundamentally an information-bottleneck.