I learned today that as it happened, Private Stewart died the very hour we had our discussion Wednesday about this portrait of him.
Look at how this fellow’s left eye is sharp, but by the time you get to the tip of his nose, or back to his ear, the focus gets soft. I can only speculate what sort of camera might have captured this image, but I bet the f-stop (focal length / aperture diameter) was 1.0 or even less. (From Bruce’s photos)