Some reptiles have very thin, slit-like pupils. This gecko even has one that makes multiple little holes.
I found that this is called a stenopaic pupil, but I wasn’t able to find much on it.
How does this work optically? How would this change the image that hits their pupils? What is the purpose of this?
*Eyes adapted for vision at night, such as the eyes of nocturnal geckos, with a large pupil and a short posterior nodal distance (here also called focal length, f), are especially affected by longitudinal chromatic aberration. As a result, light of short wavelengths is refracted more strongly and thus focused closer to the lens than light of long wavelengths. If this is not corrected for in an eye adapted for nocturnal vision, the retinal image is severely blurred. Multifocal optical systems with distinct concentric zones of different refractive powers have been suggested to correct for some of the defocus on the retina caused by chromatic aberration…
In addition, the light-adapted pupils in nocturnal geckos are different variations of vertical slit pupils. Apart from the effectiveness in shutting out light during the day, the mode of constriction of slit pupils has been suggested to be of advantage in multifocal eyes, since it allows for all refractive zones of the optical system to be functional at all states of pupil constriction.*
From here.
Tamerlane gives the textbook answer. I suspect there’s more to it.
In both cats and many reptiles, the pupils close to a slit. This has an interesting effect. Since the cutoff for the frequency of resolution (the cutoff of the Modulation Transfer Function) is proportional to the size of the aperture along that dimension, this means that cats and snakes are giving up good horizontal sensitivity in order to have better vertical sensitivity. Human pupils ()and most animal pupils) contract uniformly, staying circular, and so we have equally good (or equally bad) resolution along all directions as the illumination increases. Cats and snakes and some lizards, however, have much better resolution along the vertical (meaning thaty they can see very fine horizontal items) at the expense of reduced resolution along the horizontal direction (so the can’t see fine vertical lines). To be simplistic, cats and snakes can see horizontal mouse tails among vertical grass better than creatures with circular pupils can.
I don’t know if that’s what it’s used for, but it is an effect of closing down to a vertical slit pupil.
As for geckos, several nocturnal species have slit pupils with “notches” along the edge, although the patterns of nmotches varies from species to sdpecies. It is, indeed, called a sthenopeic pupil. Sthenopeic pupils can do two interesting things:
1.) It has extremely good MTF at certain sharply defined spatial frequencies, at the expense of virtually nonexistent sensitivity elsewhere. I’ve been trying to figure out what good this is. After corresponding with gecko experts, I still don’t see a use for it.
2.) It allows the single eye with the sthenopeic pupil to estimate distances as if it were a stereoscopic pair of eyes, but without a good idea of wshat it’s looking at. I suspect that this may be the real use of the notches.
By way of contrast, I submit the wilderbeest.
You don’t have to go that far away – Goats have rectangular pupils with rounded edges, with the long axis horizontal.
https://images.search.yahoo.com/images/view;_ylt=AwrB8o808_9Tb2AA30WJzbkF;_ylu=X3oDMTIyZmQ0dmZvBHNlYwNzcgRzbGsDaW1nBG9pZANlYTM1YTFlYzhhYjdlYjUxNTJiNzUyOWVmODFlNGU2MARncG9zAzgEaXQDYmluZw--?back=https%3A%2F%2Fimages.search.yahoo.com%2Fsearch%2Fimages%3F_adv_prop%3Dimage%26va%3Dgoat%2Beye%26fr%3Dyfp-t-901%26tab%3Dorganic%26ri%3D8&w=500&h=363&imgurl=farm1.staticflickr.com%2F116%2F263633762_44180c7cf4_z.jpg%3Fzz%3D1&rurl=http%3A%2F%2Fwww.flickr.com%2Fphotos%2Facousticdad%2F263633762%2F&size=94.9KB&name=<b>Goat%26%2339%3Bs<%2Fb>+<b>Eye<%2Fb>&p=goat+eye&oid=ea35a1ec8ab7eb5152b7529ef81e4e60&fr2=&fr=yfp-t-901&tt=<b>Goat%26%2339%3Bs<%2Fb>+<b>Eye<%2Fb>&b=0&ni=160&no=8&ts=&tab=organic&sigr=11j6s6b5k&sigb=13746f41r&sigi=11qegh8ej&sigt=10sa5b2mf&sign=10sa5b2mf&.crumb=qaEg0jdeRsQ&fr=yfp-t-901
There are lots of other weird animal eye pupil shapes besides these. Some of them I can suggest reasons for, others are still a mystery to me.
I haven’t overlooked them, but for the life of me I can’t see the optical advantage in having pupils of that shape and orientation. There’s not a big enough difference between the horizontal and vertical dimensions to make an appreciable difference in MTF, or in aberration correction, or whatever physical effect you suggest that this is in service of. And it could be that the shape is due to some other factor altogether.
Wow. I received much better answers/thoughts/links than I anticipated. Thanks.
The below is also from Tamerlane’s link. It proposes the use of the distance measurement that CalMeacham mentioned along with a theory on camouflage:
“We also investigated whether the pupil shape of the helmet geckos is related to their optical system. In NG6 the openings of the pupil at maximum constriction all fall within the inner refractive zone of the optical system ( Figure 7). We can therefore not confirm the hypothesis that the concentric zones in the multifocal optical systems match in location with the two pairs of pupils in the helmet geckos (Kröger et al., 1999). The function of the multiple-pinhole pupil remains a mystery, partly because helmet geckos are not normally active at daytime when their pupils close to four openings. Murphy and Howland (1986) suggested the multiple-pinhole pupil to be a mean for the geckos to estimate distances in bright light. An object not focused properly on the retina appears quadrupled, while an object in focus appears single. The light-adapted pupil could also, in addition to the ability to effectively shut out light and protect the light-sensitive retina, function as camouflage, as suggested already by Cott (1940). A round pupil is more conspicuous and attracts possible predators more than the irregular shape of a multiple-pinhole pupil. During the day when the helmet geckos are basking the light-adapted pupil and irregular pattern of the iris, being color-matched to the body might help them stay hidden from birds and other predators.”
