Slit pupils

I was just curious…what’s the benefit to slit pupils? You know, cats have verticle ones, goats have horizontal ones…do they really make a difference when compared to circular pupils like human’s? And do verticle ones offer different advantages than horizontal ones, and vice versa?

A cat’s eyes are very sensitive to light. Slit pupils can close down more tightly than round ones, thus a cat can go about in bright daylight without searing its retinas. I don’t know about the benefits of vertical vs. horizontal slit pupils, though.

I’ve remarked on this several times on the Board. I suspect that slit pupils are used because theyt preserve the ability to see fine horizontal detail while restricting the amoiunt of light entering the eye. Human irises close down like a camera’s – the circle in the center gets smaller. We thus spread out the loss in resolution equally in all directions, so you don’t notice it. A cat’s eye keeps the same vertical opening, while restricting the horizontal one. That gives a tradeoff – you can see very fine horizontal detail, but lose the ability to see finer vertical detail.

Think of mouse tails in a field of grass. The vertical blades of grass would get fuzzy and lose detail, but the horizontal mouse tail would still be perfectly sharp. In fact, it would stand out as a sharp detail in the field of blurry vertical grass blades. Horizontal bird tails would stand out well, too. I note that snakes, which hunt the same things, also have vertical slits. By contrast, the “big cats”, like tigers and lions, don’t have slit pupils.

There are other interesting games you can play with the pupils of other animals. But what purpose the elongated horizontal pupils of goats and some frogs serve, I do not know.

This assumes that human (and feline) eyes are diffraction limited. Do you think it’s a valid assumption? I know my eyes are nowhere near diffaction limited, even with my glasses. It’s easy to notice that my eyes have better resolution with the iris closed down (i.e. when I’m in bright light).

It depends upon what you mean by that. Some people say “diffraction limited” to mean that the only limit on what they can see is due to the diffraction of the lens, and not to aberrations. So a “diffraction limited” lens is as good as you can possibly get. I know my eyes aren’t diffraction limited in that sense – I wear glasses.

But all optical devices are diffraction limited in the sense that they cannot possibly see better than the size of the diffraction spot, and that even if you have aberrations that cause blurring, you can usually make the aperture big enough so that you can see the effect. The importance of diffraction in imaging was discovered about 165 years ago when a clergyman astronomer discovered that his horribly aberrated but large aperture lens was able to resolve stars that were closer together than his godd quality but small lens. Clearly the bad lens wasn’t “diffraction limited” in the first sense, but it still resolved better because its aperture was bigger. Sometimes you need the big aperture/small f-number more than good quality. Cats eyes are just another example.

“Search the archives, Shel, search the archives”

I meant “diffraction limited” in the sense that aberrations are smaller than the diffraction limit. In that case, reducing the aperture always results in lower resolution. On the other hand, if a lens system is limited by aberrations then reducing the aperture tends to improve resolution. Of course if you reduce it too far then the system becomes diffraction limited, and further decrease in aperture will degrade resolution. A common rule of thumb among photographers is that camera lenses produce the sharpest image at F/8 - a smaller aperture causes more diffraction and a larger aperture causes more aberration. It depends on the individual lens, of course, but very few camera lenses are diffraction limited when wide open.