Quote Originally Posted by Yrusama View Post
I used to have it backwards. I thought the Seekers of the Sun had the round pupils because they were evolved to handle more light, whereas the Keepers of the Moon adapted to limit their light intake. But I suppose the other way around makes just as much sense. Keepers of the Moon sleep during the day because they can't handle the light, whereas Seekers of the Sun sleep at night because they can't see as well in the dark. Unfortunately for Keepers, the rest of Eorzea makes heavy use of lamps and flashy magic.
The reason pupils are large in nocturnal creatures is because they allow more light into the eye. The photodetecting cells of the retina can only be so efficient, so the greater capacity to collect light at any given instant through a larger aperture helps to make up for this sensitivity limit by boosting the amount of information that would otherwise be received in low light conditions. It's the same reason telescopes are built as huge as possible, to see fainter stuff.

So yes, it absolutely makes sense this way :P A poor Keeper with fully dilated pupils walking around in the daytime would be about as happy as walking out of an eye exam after getting those glaucoma drops and not having any sunglasses...