Even when you zoom way into the eye, the detail keeps on going. He has carefully rendered the delicate overlapping wrinkles around the eye, and he has captured the redness inside the folds.
Compare to this detail of an eye from a Sargent self portrait.
High-rez file of this image on Wikipedia Commons
The painting is in Dresden at the Gemäldegalerie Alte Meister
Previously on GurneyJourney: