In The Two Towers, the elf Legolas, at a distance of five leagues, observed once, “there are one hundred and five [riders on horses]. Yellow is their hair, and bright are their spears. Their leader is very tall.” In 2014, a viral video made the claim that this was impossible, based on the equation θ≈1.22λ/d, where θ is the angular size of the Airy disk produced by a point source of light, λ is wavelength, and d is the diameter of the pupil. My idea is that, in a material with a high refractive index, λ would be proportionally less than it is in air, resulting in a smaller θ, and with it an image with better resolution.
(This post’s image and alt text are not my work; Wikipedia user Inductiveload released them into the public domain.)
The real world is seldom as simple as the maths, but several animals do see much better than humans. In particular hunting birds.
A biologist might have to chime in on this one, as it might genuinely come down to rod/cone density and other factors as well.