Nuances in the Workings of the Eye and Retina

The eye (any eye) is a miracle of evolution, which has filled its structure at every scale with an apparent intentionality that is the hallmark of complicated systems under intensive selective pressure. It is very much possible to look at almost every design feature and state a compelling reason why it's there, which makes it a rich place for the engineering-minded to find fun little design features.

This post is meant to discuss topics that are less frequently discussed elsewhere, and not often discussed in introductory courses, rather than anything that can be quickly gleaned off of common summaries. Another way to describe it is that I intend to dive a bit into the kind of fridge-logic questions you might come up with randomly after opening the evolutionary eye-fridge, particularly the ones that turn out to have interesting answers.

As such, this post is going to be a bit of a grab bag of topics, rather than a focused point-by-point breakdown of the eye. I'm still going to group topics broadly into general categories, just for organizational reasons.

One thing I will not go into is visual processing in the retina or visual cortex–even though this is a fascinating topic and one of the best understood parts of neuroscience (relative to the rest of the brain). That's simply for space considerations; including that topic would balloon this post to enormous sizes. Maybe next time.

To talk about any of this, it is necessary to have some basic grasp of the eye's anatomy and functioning. That is, you need to know the following key facts and have some sense of the diagrams below:

Basic functioning: Light hits your face, is focused by the cornea[2] and natural lens, resulting in an image projected onto the retina, which then does a ton of image processing before passing its in-progress data to the optic nerve. (which connects to the visual cortex in the brain which does the rest of the processing)

Human eye focusing light. (source)

Human eye focusing light. (source)

Now, let's explore some optics-adjacent questions that might come to mind, provided one thinks about the optics.

Anyone who has spent any time working with refractive lenses (as opposed to reflectors) is familiar with the fact that not all frequencies of light are focused in quite the same way. Specifically, short-wavelength light focuses slightly closer to the lens than long-wavelength light. In cameras, this often results in color fringing in photographs:

Notice the purple tinge on the edge of the hair. (source)

Notice the purple tinge on the edge of the hair. (source)

and results in camera-makers spending considerable effort on modifications to their lenses to reduce this effect. This is also just one of the reasons why reflectors are preferred for telescopes.

You may have noticed that you never see these effects in everyday life. So does the biological lens have a very clever system for eliminating chromatic aberration?

In a word: no. Your brain has simply learned not to see it. Human eyes often exhibit a relatively strong degree of chromatic aberration (sometimes cited as an average of 1.75 Diopters). Some mechanisms in the eye compensate for this–for instance, the center of the fovea lacks high-frequency blue photoreceptors, which then reappear in a ring around the fovea–but at the end of the day, the brain simply covers for the imperfections of its visual instrument.

As a small consequence of this, at nighttime, this combines with the optimal absorption frequency of human rods (500 nm, roughly blue-green) to cause vision to be somewhat more near-sighted. When combined with a dilated pupil (=more light into the eye, but less precise focus), this can cause serious vision impairment at night, particularly in older individuals.[3] It is more important to wear your glasses when driving at night.

So, refractive lenses work by bending light at the surface of the lens between the external medium and the interior of the lens. Leaving aside complications like multimaterial lenses, Snell's Law makes clear the degree of bending depends on the refractive index of both the external medium and the lens itself. If you've paid careful attention to the discussion above, then you can discern that there are three major points of refraction in the eye:

The cornea, where light passes from air to water.[4]

The outer surface of the lens, where light passes from water into the lens.

The inner surface of the lens, where light passes from the lens back into water.[5]

Naturally, if the eye enters water rather than air, the refraction from 1 changes drastically–in fact, it mostly disappears, since both sides of the interface are, roughly speaking, water. But recall that the cornea is two-thirds of the refraction! This is why everything looks blurry underwater unless you're wearing swimming goggles[6]–you're effectively extremely far-sighted underwater.

But now the follow-on question from the title of this section suggests itself: What about aquatic animals? They need to see too!

The answer follows naturally from optics: without the cornea providing much refraction, the lens needs to do a lot more of the work. One possibility is to use a different material, but for reasons we will discuss next section, this is probably evolutionarily difficult. Much easier to simply have a much bigger lens.

Fish lenses are nearly spherical, and this introduces its own problem: thus far all our discussions about lenses have assumed that they always focus light neatly into a single focal point. This is actually not true of lenses with a spherical surface–it requires a parabolic surface. For thin lenses like in the human eye, this is not that big a deal, thanks to the thin lens approximation[7], but for a roughly spherical lens it absolutely does matter. Rather than learn how to make parabolic surfaces, fish instead introduce a refractive index gradient within the lens itself to correct the effect.

In humans and land vertebrates, focusing the lens is done by reshaping the lens. Presumably this is more difficult for a thicker spherical lens, which may be why in fish focusing is instead done by moving the lens back and forth.

And now I've successfully tricked you into learning some geometric optics, assuming you read this far.[8]

The overall structure of the lens resembles a motif repeated frequently everywhere in biology, often when a biological system needs to generate a large-scale object that is more important for its structural properties than whether or not it is, technically speaking, alive.[9] A living layer of cells will sustain itself as the base layer, dividing occasionally. As the layer gets thicker, some cells are forced inward (in the case of the lens), and will gradually turn themselves into structural cement, entombing thems