But let's come down to earth. When a person looks down from the Sears Tower in Chicago, cars below appear as tiny as bugs, and we can barely see the people. So when we say birds see farther, we mean that at a given distance, they see things in much greater detail. Eagle eyes have several features that give them clearer vision than humans have. One is the little spot in the retina called the fovea.
Primates including humans, and all birds, have a fovea.
When a person or eagle is looking at something, we turn our head and eyes so that the light from the image hits our fovea. Humans have such great vision because our fovea has about 200,000 rods and cones packed into each square millimeter of the fovea. That's about the same number of visual cells as the finest computer monitor has on its entire screen when set at its highest resolution. The resolution for a person would be similar to setting a computer's screen at a much lower resolution. Eagles can't read an eye chart to show us how good their vision is.