Excerpts from An Immense World by Ed Yong.Chapter 2 is about light and vision.
Sensitivity and resolution seesaw against each other. The more photoreceptors you pack into a small space, the higher the resolution. But each photoreceptor collects light over a smaller area and is thus less sensitive. An eagle can spot a far-off rabbit in daylight but its acuity plummets as the sun sets. Conversely, lions and hyenas might not be able to resolve a zebra’s stripes at a distance, but their vision is sensitive enough to hunt one at night.
An animal’s visual acuity is measured in cycles per degree. One degree (out of 360) is roughly the width of your thumb when you hold it up at arms length. You should be able to paint 60 to 70 pairs of thin black-and-white stripes in one degree of your visual field and still be able to tell them apart. A human’s visual acuity is 60 to 70 cycles per degree (cpd).
Humans outshine almost every other animal at resolving detail. Birds of prey are the only animals whose vision is substantially sharper than ours. The current record belongs to the wedge-tailed eagle at 138 cpd. It can spot a rat from a mile away.
Only primates come close to our standard. Octopus (46 cpd), giraffes (27 cpd), horses (25 cpd), and cheetahs (23 cpd) do reasonably well. A honeybee’s acuity is just 1 cpd. Around 98 percent of insects have vision that is even coarser.
A heron’s visual field covers 180 degrees in the vertical. A mallard duck’s visual field is completely panoramic, with no blind spot.
Humans have an acute zone, where vision is sharpest, that is a bullseye - a round spot in the center. Many birds of prey have two acute zones in each eye, one that looks forward and one that looks out at a 45-degree angle. The side-facing one is sharper. When a peregrine falcon dives after a pigeon it doesn’t plunge in a straight line; it flies along a descending spiral to keep the target locked in its side acute zone.
In birds, the right eye (left brain) is specialized for focused attention and categorizing objects. The left eye (right brain) deals with the unexpected; it is better at scanning for predators and reacting quickly.
CFF is a measure of how quickly the brain can process visual information. It stands for critical flicker-fusion frequency. Think of it as a frame rate - a point at which images blend into the illusion of continuous motion. For humans, in good light, the CFF is around 60 frames per second (or hertz, Hz).
Cats are slower than us (48 Hz), and dogs are slightly faster (75 Hz). Scallops (1 - 5 Hz), toads (0.25 - 0.5 Hz), and leatherback turtles (15 Hz) are quite slow.
Swordfish (5 Hz) can heat up their eyes and brains to boost the speed of their vision by 8 times.
Many birds have fast vision, maxing out with the pied flycatcher (144 Hz), a small songbird, which is the fastest of any vertebrate that’s been tested.
Honeybees, dragonflies, and flies (200 - 350 Hz) process images incredibly fast, in part because the signals don’t have to travel very far. The way to catch a fly is to move towards it slowly. If you’re slow enough, you’ll just be part of the background.
For a kid-friendly book about animal eyes, check out Eye to Eye by Steve Jenkins.He makes unique art for his books by cutting and arranging paper.
People with excellent vision can distinguish a zebra’s black-and-white stripes at 200 meters away. Lions can only do so at 90 meters, and hyenas at 50 meters. And those distances roughly halve at dawn and dusk, when those predators are more likely to hunt.
In invertebrates, it’s called an acute zone. In vertebrates, it’s called an area centralis. If that area is inwardly dimpled, as it is in our eyes, its a fovea.