What does it mean when wildlife biologists say that “an eagle can see a rabbit in a field from miles away”? Does this imply that their vision is automatically more zoomed in? Do they have better than 20/20 vision, or is their vision simply clearer?
In: Biology
One way to visualize this is to think about staring at a computer monitor and thinking about the smallest amount of movement it can represent.
Imagine the whole screen is white and there are 100 black dots exactly 1 pixel in size representing blips a radar is picking up. For fun, let’s say this is an AWFUL monitor and 1 pixel is 1 square centimeter.
So what should happen if, in terms of the position of the dots, 2 of them are only 0.5 cm apart? Ignoring some fancy tricks we could do, our monitor has to fudge it a little. Either it incorrectly displays the two dots on the same pixel or it has to incorrectly display them side-by-side. Makes sense. It can’t light up a pixel “in between” two pixels.
Now imagine there’s a dot that, from the radar data, has moved 0.5cm. Again, the monitor has a choice: incorrectly leave the pixel alone or incorrectly move the pixel.
What we’re learning here is with this monitor, we can’t do a good job seeing things smaller than 1cm square.
If that is how human eyes see the world, eagle eyes are like if we replaced the monitor with one that has pixels 0.01 cm square. They are much smaller! So now if a dot moves 0.5cm, the movement is visible. Or if two dots are even 0.1cm apart, you can tell.
Now, our eyes don’t really see in “pixels”. But what happens is if two things get close together beyond the extent of where our eyes can focus, they blur together. If we try to look at a rabbit a mile away, and there is grass near it, we have a VERY hard time because our eyes only see blurry stuff that’s about the same color. The rabbit has to move for us to start to sense something’s up. If the same rabbit and the same grass were only 100 feet away, we’d be a lot more likely to be able to tell the details.
When the eagle is looking at the rabbit a mile away, it can still see the grass *and* the rabbit. Both its eyes and its brain are set up to look at things that far away with more detail than we can. It’s *like* they have binoculars, but technically that’s a bad comparison because when we zoom in with binoculars we see more details but also a smaller field of vision. Eagle eyes see both more details AND a wider field of vision.
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