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I know that this would, perhaps, be biologically impossible, but suppose it's possible, how would such person see?

I know that cats have a vertical adjustable pupil and goats have a horizontal one, mixing both would be something kind of like this (I made a trashy editing on a cat eye just to demonstrate):

enter image description here

It wouldn't be permanent, i.e., the pupil would be able to adjust itself from a cross-shaped pupil to a round one.

A pupil like that would offer any benefits? What would be the disadvantages?

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  • $\begingroup$ How would this work mechanically? Contracting the iris edge would pull the flaps out. Are they held in place by transparent (nope) or thin (maybe) muscles? $\endgroup$ – John Dvorak Jun 25 '17 at 15:23
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In this answer I noted reasons why an optical instrument (that happens to be part of a living creature) would want a large aperture. That made me think about slit pupils at the time, though I did not mention it in that post.

Depth of Field

Do you know how auto-focus works on dSLR cameras, and the “split prism” people used before that? The phase matching works by taking light from each side of the aperture separately and comparing the separate images. This only works if the aperture is in fact large enough to have separated sides.

Now arguably a smaller aperture provides a sharper image already. But the wider aperture allows for accurate ranging: the determination of distance to an object, rather than just seeing it sharply.

This is the effect aluded to by the articles linked by Mołot’s comments.

Now consider this question on the Photography SE: What's the difference between cross-type autofocus points and regular ones?

cross-type sensor is simply two linear sensors crossing each other (making a cross shape), which is superior because it can work with both vertical and horizontal light patterns.

A cross-shaped pupil allows for the angular resolution of a large aperture, while masking off the excess light from the large area of such an opening.

Cameras find it advantageous to use cross shapes rather than slits, so they can focus on a larger variety of objects. An animal may find the same advantage.

phase or contrast focus

Note that the principle is the same regardless of whether the light from each edge is sensed separately. If separate sensing is used, the features can be matched and this is called phase. For a single image taken with an extended aperture, you simply judge the sharpness of the image. In either case, we exploit the physics responsible for reducing the depth of field as we increase the physical size of the aperture.

Diffraction Limited Optics

In the case of an instrument working at full aperture to get angular resolution, the implication is that a larger aperture can produce a sharper image (of the in-focus item). In the case of synthetic aperture instruments the cross-shape ought to provide the additional information. However, I don’t know if a simple projection onto a retina would be sharper or not: the blurring due to diffraction might be caused by the narrow parts and having wide parts as well doesn’t un-blur it.

But you could imagine that the optical system behind that alien aperture is not like a mammal’s. It could be something designed to indeed have a high angular resolution needing a mimimum physical aperture size.

Wierd Eyes

From Mołot’s links, we see that some animals have various special features in the eyes, like multiple fovia or some kind of separate zones for different colors. The shape of the pupil might not make sense if you assume that the eye is otherwise just like ours, but is part of a complete system.

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  • $\begingroup$ Hmm, so such pupil wouldn't provide any really useful benefits, right? $\endgroup$ – Yuuza Oct 31 '16 at 2:54
  • $\begingroup$ I updated my answer. $\endgroup$ – JDługosz Oct 31 '16 at 2:57
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This is, indeed, biologically impractical. Pupils are controlled by small muscles, which cannot "push", only "pull". The muscles to pull a cross-shaped pupil into a narrower cross would need to cross the centre of the pupil, which would definitely interfere with vision.

Ignoring that problem, there don't seem to be major advantages for humans. The Wikipedia article on pupils has some hypotheses on why some animals have non-circular pupils, but the summary is that we don't yet understand this in detail. Slit pupils seem to be mostly found in animals that are active at dusk and during the day; they can close more completely than circular pupils, which is helpful for creatures with very sensitive retinas.

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  • $\begingroup$ This is inaccurate, take a look at this article. Plenty of examples of eyes that do form weird, irregular pupil shapes when contracted. $\endgroup$ – Vaesper Oct 30 '16 at 21:36
  • $\begingroup$ Fair enough, but how do those pupils contract? $\endgroup$ – John Dallman Oct 30 '16 at 22:30
  • $\begingroup$ @Vaesper yes, but question includes morphing between round and cross. All found in nature have only one shape at given aperture. $\endgroup$ – Mołot Oct 31 '16 at 6:51
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    $\begingroup$ @Molot I don't see the requirement for same aperture between round and cross, the way I read it was "round when fully dilated, cross when constricted". May not be possible to have a perfectly round pupil with such an irregular shape but something in the direction would be plausible imho. $\endgroup$ – Vaesper Oct 31 '16 at 8:22
  • $\begingroup$ @Vaesper Well, both interpretations may be valid, and yours makes it plausible. $\endgroup$ – Mołot Oct 31 '16 at 8:40
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One thing that a weird-shaped eye could do is gauge distance a particular way.

If you imagine the cross-pupil closing entirely and a pinhole forming on each tip of the cross, the eye would see several images at once.

Some ambush predators use something like this to know when to strike; when the images line up the prey is in at the perfect distance for the predator to strike. Or, they would be able to look in several directions at once, with the same eye. Or, just gauge depth with just one eye.

There's also W-shaped pupils (cuttlefish) that do something similar; the prey is at the right distance when the image isn't distorted in an n or u shape.

An X-shaped pupil isn't very far fetched I'd say. It could evolve from something like the horse's corpora nigra, or something like expanding flanges that form the crescent-shaped pupils of skates.

Source: http://www.koryoswrites.com/nonfiction/the-functions-of-different-pupil-shapes/

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Perhaps this pupil shape allows for polarization of incoming light, or acts like a slit to allow for diffraction. The retina could have a pattern of very specialized photoreceptors that use the diffraction pattern for very high sensitivity vision. So rather than have a homogenous mix of photoreceptors across the retina that get exposed to plain light from a round pupil, the retina exploits diffraction to have photoreceptors of specific wavelengths set up in a pattern based on the diffracted wavelengths. Slit light diffraction and polarization interactions are discussed here which I link simply because it is from my alma mater :)

Why would this be helpful? Maybe for increased sensitivity to specific wavelengths of light, like if this animal is a predator at dusk when most of the light is reddish, or in caves with phosphorescent lighting of a narrow wavelength. It still needs general vision when out and about, but for "hunting mode" it can form various slit patterns to see in different types of light.

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