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So, dragons. They're omnivores and, while they would be potentially formidable hunters, mostly scavengers. This requires them to be more sparing with their energy.

Still, they fly and flight needs good eyesight and good eyesight needs this:

eagle eye to human eye comparison

Well, if I want my dragon to be able to track your movements with their eyes without moving their head (you know, like a human) or quite literally look down upon someone, these eagle-eyes won't cut it.

But now I'm stuck. I want my dragons to be able to spot hares from 5 kilometers away but without losing the ability to roll their eyes. How can such system work?

I'd also prefer to keep the count of the eyes to two and the size of them as small as possible.

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    $\begingroup$ Whoa, eagles cannot move their eyes inside their skull! I never knew! nationaleaglecenter.org/eagle-eyes $\endgroup$ – PipperChip May 7 at 21:37
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    $\begingroup$ @PipperChip Y.A.N.A with that. (the acronym is from Doctor Who) $\endgroup$ – Mephistopheles May 7 at 21:39
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    $\begingroup$ Just two eyes? There goes my answer. (And the author thought those were gemstones embedded among the scales...) $\endgroup$ – Jedediah May 7 at 21:40
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    $\begingroup$ Eye movement does not necessarily imply the rotation of the eye itself in its socket. Other arrangements are possible. Here's a YT clip showing how this works. $\endgroup$ – AlexP May 7 at 23:32
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    $\begingroup$ @AlexP Why aren't you posting that as an answer? $\endgroup$ – Halfthawed May 8 at 1:39
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You see, apparently the main reasons behind that lack of movement is the tight fit that the eye is in their skull and the presence of a sclerotic ring. The eyes of the eagle can be as large as a humans despite them having much smaller skulls. The owls took this eye to head ratio further, to the point that the back of their eyes can be seen from the inside of their ears. Here's the skull of a harpy eagle with the sclerotic rings and an informative image about them:enter image description here

enter image description here

Now, let's look at what makes their eyes so good:

1-size

The size of eagle eyes, disproportionately large compared to the skull, is a good reason. Bigger eyes mean more space for photoreceptors, which work in a much similar way to pixel count, the more receptors, the better the image. That's also a reason why we believe T-Rex had great eyesight, with their eyes allowing for binocular vision and being the size of oranges. Additionally, eagle eyes have a much higher number of cones than us.

2-the eye itself

Not only eagles have a convex fovea, which we assume helps them focus light better, they have two foveas in each eye (we have 2), one responsible for good monocular vision and the other for binocular vision. In addition, both their lens and cornea can change shape (we only have the lens change), allowing them for even better focusing.

They also have a pectan, but I'll elaborate that further down.

Now, regarding the dragon.

So, we want a dragon that has eagle like vision but can move its eyes around. For this answer I plan to focus on a more classic look, without the need things like tentacles in their faces or "popped out" eyes like an octopus (though I recommend looking at those, due to them having their photoreceptors inverted in relation to ours, they have no blind spots).

The main difference would have to be a large skull. If your dragons have large enough skulls, they can fit larger eyes, meaning more space for receptor cells and better vision (remember that big eyes are useless without also having brain space dealing with the information sent by them. See how eagles have such well developed brain parts regarding eyesight that they have almost no space for smell and taste). Additionally, the eye sockets will likely need to be fairly larger. See how the harpy eagle barely has space between the scleral rings and the sides of the socket. In order to have eye movement, you'll need sockets large enough to be able to contain the eyes (which might be easier to do for the eagle's more roundish eyes than for the owl's eye tubes). I would recommend keeping the rings though. Even if they are a bother to move the eyes, like the pic said, despite other important functions, they're attachment points for muscles responsible for focus, so it safe to assume they play an important role, likely in their shape changing lenses.

Lastly: circulation. One thing about managing the eye is that everything that isn't a photo reception cell in the eye means there's a place with less receptors. That's why eagles have very little bloodvessesls on their eyes (unlike us, look in the mirror and you can see the blood vessels in your eyes). This is likely possible due to a structure called pecten, which, while we don't know for sure, is assumed to be the responsible for the nutrition of their eyes without the need of bloodvessels that'd take space.

So summing up, to have a more classic-looking dragon with eagle vision, your animal will need:

  • eyes decently big to accommodate more photoreceptors.

  • a skull with large enough eyesockets in order to have space for the eyes to move around. You'll also need space to fit the muscles responsible for moving the eyes (humans need 4 groups of muscles for their eye movement ability).

  • convex foveas (having only one should be enough, unless you also want them to have good monocular vision).

  • lens and cornea capable of changing shape (likely means the need of a sclerotic ring as attachment point for some of these muscles).

  • special nutrition methods which allow for the lack of blood vessels in the eye (maybe a pecten).

So, yeah. You can do fine with only 2 eyes, but having them as small as possible is a no go, simply cause we can't quite shrink the structures responsible for receiving light, so to have more of them, your dragon will need to have big eyes. Additionally, as you want these already large eyes to be able to move around, you'll need even larger sockets than they'd already have with fixed eyes. The only way for you to make their eyes look smaller without reducing their capacity is making the dragon itself bigger.

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Eagle eyes are, indeed, fixed in the head and unable to move, but the important features of the eye that allow clear images of distant objects are only the power (thickness/curvature) of the lens and the density of the photoreceptors on the retina. You can see from the images in the question how much more focusing power the eagle lens has to work with, but it comes at the cost of an eye that is roughly the same size as a human's! Those eagle eyes can take up half their head. I'd hypothesize that having such a large eye in such a tiny bird head is why they don't move independently and instead rely on the eagle pointing their head. In this case, your dragons should be able to have equally large eyes with equally high focusing power but still plenty of space in their huge dragon head for the eyes to move around like a human's.

Some other interesting notes about raptor retinas:

  • Their foveas, the highest photoreceptor density part of the retina where the image is formed, are higher than our own, contributing to their excellent visual acuity. The parts of the retina outside the fovea have much lower density in both raptors and humans, which you can think of as your peripheral vision (much worse quality than the center of your field of view).
  • They have two foveas at different locations that use different parts of the lens to see different fields of view at different magnification. The center fovea looks straight ahead at relatively closer objects, and the second looks out at 45 degrees to distant objects with high focusing power through the lens. This is maybe helpful with the fixed eyes as well -- human eyes rotate to place the image of the world on the single fovea where photoreceptors are most dense. Raptors at least have two foveas to choose from when tilting their head to look at things.
  • Humans refocus to different distances mainly by changing the shape of the lens, but eagles apparently also manipulate their cornea, which, again, is quite large in the diagram, allowing nearly parallel rays from distant objects to be turned in and focused onto the retina.
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  • $\begingroup$ Eagles, like most birds, also have much more flexible necks than humans, which more than makes up for the lack of movement in their eyes. $\endgroup$ – Darrel Hoffman May 8 at 17:27
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Have their eyes atop of a structure resembling snail eyes, scaled up for the dragon eye size.

snail

Moving the protuberance around is less energy consuming than moving the entire head, plus the very protuberance can offer the eyes additional protection when needed, by simply retracting the eyes inside the body.

Additional point: it's cool!

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Transparent nictitating membrane, with a variable lens like properties.

Fine fibers like those in the eye's crystalline can thicken/thin the membrane on localized areas (perhaps acting in vertical slices), bringing other things into focus. The nerves responsible with moving the eyeball in other animals will take the responsibility of "focus shifting by varying the thickness of the nictitating membrane".

It's like a sophisticated adjustable fluidic lens, but with a protein membrane instead of a fluid.

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Build a wrapper.

Eagle eye wrapped in a sphere

The idea is fairly simple: you put the "inner eye" into a spherical "outer eye", which allows the normally immobile eagle-type eye to freely rotate as if it were a human eye.

The only downside with this method, is that it wastes space. Unfortunately, I have no idea what kind of evolutionary pressure would have your dragons evolve 2 distinct layers of eyes, but as long as your dragons have a big enough head, this shouldn't be too much of a disadvantage.

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A few other things to note:

Diffraction

There is an optical property that when light passes through a small opening, it gets blurry. A human eye, at half a mile, cannot distinguish a 6 foot human as more than a single point. (I haven't seen the math since about 1980, so I don't remember it at all.)

Refraction

Another optical property, used in lenses. This has the opposite problem: the larger the opening, the blurrier things are.

Microtremors

In the human eye, the eyeball is constantly jumping back and forth. Eagles don't do this. When the eye doesn't move, the nerves all drop back to their rest state, and everything disappears. Eagles use this to hide the scenery, so that only the rabbit moving across the field is visible to them. (Some details at https://en.wikipedia.org/wiki/Fixation_(visual))


The net result: You may need to have rather large eyes if you want to distinguish things at 5 kilometers, but then in will be messed up by refraction. What you might need is what astronomers do: Reflection. Put a couple tiny eyes on the snout pointing backwards, and a pair of parabolic mirrors where other things have eyes. Microtremors -- is up to you (though maybe it can turn them on and off?).

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