I don't understand the actual science well enough myself to explain, but my understanding is that there are lots of limits to do with optics, the behaviour of l

I don't understand the actual science well enough myself to explain, but my understanding is that there are lots of limits to do with optics, the behaviour of light over such distances, the wavelike nature of light, etc. See https://en.wikipedia.org/wiki/Diffraction-limited_system https://en.wikipedia.org/wiki/Diffraction-limited_system for example.