Explanations for pupil contraction

OK, peoples’ pupils contract to smaller points.
Domestic cats pupils contract to vertical slits.
*Tigers’ pupils contract to smaller points.
Horses’ pupils contract to horizontal bars.

Is there any advantage to the way a pupil contracts?

I can speculate, but I’d prefer some real answerws.

*I’ve read this one, but have not personally observed it.

Mjollnir, I think you’re wrong on horse’s pupils – they contract to circles, as do those of Tigers and Lions. The pupils of Goats contract to horizontal ovals, though. The pupils of snakes, like those of cats, go to vertical slits.

There are a great many weird shapes ouyt there – some sharks and rays have pupils that contract to annular shapes – there a central obscuration called the operculum. Some lizards have horizontal slits.
Now, what’s it all about?

I’ve never seen a proper explanation, and I’ver thought of writing about this. It’s clear to me that it’s a matter of the Modulation Transfer Function of the eye, viewed as an imaging system. (Alternatively, you could look at it as a matter of the size and shape of the best focused spot).

The pupil closes down to control the amunt of light entering the eye, just as a camera lens does. But this closing down also inevitably affects the quality of imaging. The larger the aperture, the finer the details you can make out. Optical engineers call the frequency response of an optical system the Optical Transfer Function. The real part of this is the Modulation Transfer Function, or MTF. For most systems, the OTF is completely real, and so the OTF = the MTF.

The MTF is a measure of how faithfully the visual system conveys the image. It’s characerized as a fubnction of spatial frequency – how well can you image a certain pattern of alternating light and dark bars. That’s why the USAF imaghing target uses all those three-bar targets, and your eye doctor has you look at charts with “E” facing in different directions, at different sizes.

The larger the aperture, the larger the spatial frequency you can view. When your pupil closes down, or you “stop down” the aperture on your camera, the aperture gets smaller, so you can’t see some of the higher frequencies anymore.

The housecat has the same problem, but instead of closing the eye down equally in all directions, it closes it horizontally, leaving the vertical dimension untouched. As a resultm, the cat can still see fine horizontal details (like mousetails, I guess), butm, because its eye closes down even more in the horizontal direction than ours do, the cat can’t see fine vertical lines as well as we do. As a result, the details of a field of grass tend to blur out. So the cat sees horizontal mouse tails, but can’t see the vertical grass so well. It’s a perfect situation if you want to see and catch mice. Snakes, which also catch mice, have the same advantage.

I don’t know if this system evolved to catch mice, of course, I just use this as an example. Bigger cats, like Lions, have round pupils. I guess it’s because zebras and antelope don’t have long horizontal tails.

As for the annular pupil of the sharks and rays, in an ocean environment the usefulness of up and down is not so great, and your prey can take any orientation. The annulus lets the shark or ray have extremely good vision at one particular spatial frequency in all orientations – presumably one characteristic of its prey. The MTF at that frequency actually exceeds the “diffraction limit” that would be imposed by an unobstruxcted pupil.

For the life of me, though, I still haven’t been able to figure out why some creatures have a horizontal slit pupil.

New Scientist The Last Word article: Why do some animals have a noncircular pupil?
A thread: What does the world look like to a cat?