As Shakespeare said, "The eyes are the window to your soul". Not all portraits include a persons eyes, but when they are in the shot, you generally want them in sharp focus; in fact, you generally want both eyes in focus...and then you get creative!
This model, Leah, has gorgeous eyes and I wanted to really draw attention to them in this shot, by using selective focus and holding the sharpness too one eye only, there is no question where the viewer will look. ISO 100, 1/100 sec at f2.8, 70-200mm lens.