The unique visual system of the eagle

When it comes to vision, eagles are widely considered to be at the pinnacle of visual evolution. And this reputation is well-earned; that they can spot their prey from distances that would have us reaching for the binoculars strongly implies the acuity (the ability to resolve fine detail) of their eyes far exceeds our own. Indeed, in comparable experiments, eagles are able to discriminate detail that is almost twice as fine as the finest detail that we can see. So what is it about eagle eyes that make them so keen?

There are two ways an animal can increase the resolution of its eye; they can reduce the space (see figure 1) between their photoreceptors (cones and rods) and they can increase their eye size. Due to the physics of light, the absolute minimum separation between cones for an eye to function correctly is 2µm (0.002mm).