When we look at the eye of an eagle and it's position we see that he has rather large pupils which cause minimal diffraction (spreading) of the incoming light. The masking-alpha-channel between the eyes, created by their beak, is very large, the placement of the eyes is not sideways as with most birds but frontal.

As a hunter, the eagle catches moving prey like rabbits, mice and fish, he spots and follows them concentrated from a high distance, while he is flying/moving. The focus/concentration level of these birds is enormous and can't be at any moment blurry. To conclude, the proportional masking-alpha-channel is tuned for great distances and high detail.