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Currently, I'm creating an alien species with two eyes, but two pupils in each eye. Though I'm not sure how it would affect its vision, whether negatively, positively, or both, so I've been looking for an answer.
I previously asked this in the Biology community, though I was suggested to turn here instead.
From the perspective of a human, the results are bad.
But, if the eye is designed for something non-human (like a goat, frog, cat, or...) like two holes, then there could be some benefits.
If you have a two-chambered retina, it would allow you to receive something different in one compared to the other. This would only make evolutionary sense if what was coming through the pupil couldn't be dealt with by the construction of the retina. In other words, if you add cones to detect infrared light, that would NOT justify two chambers.
On the other hand, a second chamber would be useful for full-spectrum sight (or, perhaps, "normal" spectrum sight) if you're in a binary star system and one of those honkers is really, really, really bright. The second chamber would be more acute than simply shading a single chamber.
Or, if you really color outside the lines, the second chamber might heighten contrast, or heighten saturation, or some other Photoshoppy effect that would give them an advantage over just seeing plain old color.
So, if the eye is a two-chambered retina and the muscles of the eye are designed to manipulate two pupils, we have an ultra-cool solution with an amazingly high geek factor.
If, on the other hand, you're just thinking about human eyes with two holes. Nope, there's no advantage at all. In fact, it's all downhill.
It's possible and the effects should be positive.
It's possible to have two pupils in the same eye with each adding their own benefit. As linked by Renan, the four eyed fish has four pupils in two eyes (so it's a bit of a misnomer). The upper pupil of the eye is adapted for vision in air while the lower pupil for vision in water. That is, the refractive index of the lens in front of each respective pupil is adapted for the different refractive index of the medium they're seeing in. It also has two separate retinas inside a single eyeball, one for each pupil.
For your own creatures it would likely work in a similar fashion, for vision in different mediums or even different colors or spectra (assuming it als has two retinas).
If you introduced a creature with multiple pupils that had no evolutionary advantage, a reader may question if you really thought it through at all.
Not sure about how this would affect an alien, but in humans this is a medical condition with the following side effects:
Although, Pliny the elder wrote that it may give you magic powers, but that has yet to be determined
There are people of the same kind among the Triballi and the Illyrians, who also bewitch with a glance and who kill those they stare at for a longer time, especially with a look of anger, and that their evil eye is most felt by adults; and what is more remarkable is that they have two pupils in each eye.
Your creature occupies a middle trophic level. It is a predator but is preyed on by larger predators.
When in the role of predator it uses the vertical pupil to maximize depth perception and perform accurate ambush attacks. When under threat by predators it constricts the vertical pupil and uses the horizontal pupil to maximize field of view while looking for danger.
https://en.wikipedia.org/wiki/Pupil
A 2015 study[16] confirmed the hypothesis that elongated pupils have increased dynamic range, and furthered the correlations with diel activity. They showed that vertical pupils enable ambush predators to optimise their depth perception, and horizontal pupils to optimise the field of view and image quality of horizontal contours.
Depicted: vertical predator pupil (cat), horizontal prey animal pupil (goat).
They have the best eyes in the animal kingdom, with not two, but three pupils in each eye. Humans have binocular vision - we perceived depth by focussing two pupils on the same point. Mantis Shrimp have trinocular vision giving them better depth perceptions than humans using only a single eye. Not to mention that they see in the infrared and ultraviolet range and discern polarized light in ways that we can't. These phenomena aren't a result of multiple pupils, but rather because of a larger number of photoreceptors - twelve instead of the three that humans have. In short, their vision is something we can't quite even imagine.
They have other amazing qualities, like being able to create shockwaves underwater that stun prey even if their bullet-fast striking appendages miss. Oh, and the Department of Defense is studying the structure of its shell for the sake of designing better body armor. But I digress.
EDIT:
Correction by Samuel confirmed - their eyes are indeed compound, like those of insects, so their "pupils" that we see in the photo are an optical illusion - which I think is still pretty cool. Details in this article, which also tells how they can distinguish between right- and left-circular polarization of light, and may even use this as a communication method. Their vision is still trinocular, however, as each eye has three distinct sections.
I wasn't going to, but LSerni in the comments already did. Obligatory link to The Oatmeal, though the notion of them being able to see so many colors was disproved - many of the extra photoreceptors are used for some of the above purposes.
I'm no biologist, but a pupil typically isn't just the aperture, it's also the area the Lense fits to.
If you have two pupils, would you not have two lenses? In which case having radically different lenses would enable very different behaviours of the eye without sacrificing the benefits of one over the other.
Eg: Extremely good distance vision AND extremely good close vision in the same eyeball without having to refocus dramatically. Sort of a built in bifocal lense.
