This is the eye of the American kestrel. According to numerous reports, this eye is capable of seeing an insect that is only 2mm long from the top of an 18m tree.
This being the Internet, however, it is uncertain as to how accurate this figure is, so I went digging for experimental detail. I was unable to find this particular one. However, there is a study by Matthew F. Gaffney and William Hodos in which they anesthetized a kestrel and poked stuff in its eyes. (More scientifically accurate detail here : http://www.sciencedirect.com/science/article/pii/S0042698903003043 ), in which they found that there is (potential) conspiracy behind avian vision acuity studies, and data does not always match the reality we live in. This (possibly) represents a scam in which bird scientists bilk the public out of hundreds of dollars to further promote their own research. It’s a travesty (maybe) and we should (perhaps) do something about it.
Alternatively, maybe different birds just have different eyes.