Beemer (dr_tectonic) wrote,


I fell out of the habit of posting while work was so busy last fall, and now I've been feeling like I need to Say Something Important about my life or the world when I post, and don't know what that is so I don't. Which is dumb, so here's a post about something cool I recently learned about.

So it turns out that humans (and other primates) have better visual acuity (ability to see detail) than most of the animal kingdom. Some birds, particularly raptors, have us beat, but we can see more detail than all the other mammals. We can also see more colors: most mammals only have two-color vision, but we have three-color vision.

BUT, it turns out that cats and dogs can see a bit into the ultraviolet range! The lenses of our eyes are a little bit yellow, and screen out the shorter wavelengths of UV light. (Which means that if you ever have cataract surgery, in between when they remove the old cloudy lenses and implant new synthetic ones, you too will be able to see ultraviolet for a short while. Apparently it's a sort of washed-out bluish-purple color.) That's not true of cats, dogs, and many other mammals; their lenses transmit a fair chunk of UVA -- about 60% for cats & dogs. (Here's the paper about it.)

So humans can distinguish more colors (three-color vision), but over a smaller range. That's kinda weird, and it gets weirder. There are two flavors of shortwave-sensitive (blue) optical pigment: one is more sensitive in the UV range, and the other is only goes up to violet. We have the violet one. But the UV-sensitive one is evolutionarily older. Which means not being able to see in UV is actually an adaptation. So the question is, why? Why would it be advantageous to lose sensitivity to a greater range of colors?

And the answer is apparently that there's a tradeoff between acuity and sensitivity. Screening out short wavelengths reduces Rayleigh scatter and chromatic aberration, which give you blurrier vision at the blue end of the spectrum. And all the critters that have yellow lenses are diurnal. Nocturnal and crepuscular critters, on the other hand, are more concerned with absolute sensitivity under low-light conditions, so they have clear lenses as well as features like a reflective tapetum lucidum at the back of the eye, which increases overall light sensitivity at the expense of fine resolution, and a higher rod to cone ratio.

So you can see well in the dark, or you can see a lot of detail, but not both.

Also, cats are apparently kinda near-sighted because they don't have any muscles to change the focus of their eyes. Which would explain a lot, honestly.

(Now I'm thinking this probably means that D&D races that don't have low-light vision should get a bonus on seeing fine details and at distance. And does that mean darkvision is just echolocation? Should we be imagining dwarves and orcs wandering around underground going "meep meep meep" to figure out where the walls are? That's hilarious and I love it.)

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