Pupils dilate in darker lighting, the bigger the aperture, the narrower the depth of field get. if you have old eyes that aren’t as good at focusing anymore, narrowing the aperture will cheat that and allow sharper focus, but you need more light to do it.
You can see this if you have an old SLR camera with DOF preview, and focus on a scene with object both close and far. As you close down the aperture (move it to a higher number say 2->16), you’ll see more and more of the scene simultaneously in focus, but it will also get darker and darker.
That’s why old people make rooms brighter. It’s easier on their eyes if they wish to see things in focus.
Edited for spelling
