sharp eyes like those of a squirrel.

The eye of a hawk; hence, a sharp or keen eye like a hawk's.

slang. An eye with a keen or penetrating gaze. Now rare.

(a) a squint-eye, (b) a sharp or piercing eye.

in quots. figurative, an eye as keen as that of a lynx.

The eyes of an insect or other bug (bug, n.2 1). Often in similative use, with reference to their round or bulging appearance.

the ability to record or recall detailed impressions of what one sees with photographic accuracy; (also) a person capable of unusually detailed or…

(a) an eye able to see or adapted for seeing in the dark (usually in plural); (b) U.S. = chestnut, n. 5.