The retina is finally turned the right way around
If ever you have poked around in the Evolution vs Intelligent Design debate, you will have noticed the irreducible complexity argument that says that the eye is much too complex to have evolved, so it much have been designed by some intelligent being.
Well that supposedly "intelligent" being then did something really stupid:
They mounted the human retina backwards
The light sensitive parts of the retina are to the far right. Lots of junk that light has to pass through before getting there. (Image source)
In an evolutionary mishap, the human retina became such that the light sensitive parts of it is pressed up against the wall of the eye, while the nerves come out inside the body of the eye. This is inconvenient for several reasons, including getting a blind spot in the eye, and having much less light sensitivity than we could have had.
So the easiest way your magic mutation can give us much better vision than before is:
- Flip the retina the right way around, placing the light receivers facing the light instead of the back of the eye, and no nerves in the way.
- Do like on cats and dogs, and add a tapetum lucidum, that is to say a reflective layer right behind the light receptors. This makes more light available to the receptors.
Also we have another really silly peculiarity. In low-light conditions, our most light-sensitive cells are not in the center of our field of vision but on the edge. This means that we can see things "out of the corner of our eye", but as soon as we turn to look at them, they go away.
- Make the most light sensitive cells be gathered in the center of our field of vision, not on the outside of it.
And in order to protect the eye and make it less painful to experience this brighter light, especially in sunlight...
- Make the cornea and the lens of the eye be a filter for ultraviolet radiation.
This is because it is mostly UV that hurts the eye in strong light conditions, not visible light, or at least not quite as much.
All of these things can happen without any visible change to the human appearance...
...except that instead of getting red pupils in a photograph, they will get green dittos.
Dogs, cats and more get green eyes when photographed thanks to the tapetum lucidum.
(Image source, License CC BY-SA 3.0)