The level of detail in the portrait is impressive, right down to the fly that sits at the base of the picture frame. This is considered a notable example of
trompe-l œil ( deceive the eye ), a technique that creates the optical illusion of three dimensions.
A Khan Academy lesson examines the distinguishing features of the Christus painting, including this interpretation from Dr. David Drogan, a specialist in Renaissance art and frequent lecturer at the Met. The inclusion of a fly is maybe to remind us that life is short, said Drogan. That mortality is always with us and death is always around the corner because flies, of course, are normally found around things that are decaying. So this fly at the bottom is to remind the viewer that even though this person might be in the flower of his life and looking very vital and strong and living, that always death is around the bend.