Hunters' pupils will shrink when they see an enemy.
"The pupil of your eye can expand as much as 45% while looking at someone you love."
Hunters when they see Gordon:
i thought those were weapons and the small spheres near them were the eyes, so i assume the shrinking would be some form of accuracy adjustment
It’s a natural body reaction for direct focus, much how human beings will also squint in order to adjust their focus and attention on a specific coordinate.
My guess is that for Hunters, their optics do the same to give them greater accuracy for firing Flechettes and successful ramming attacks against targets.