Did he eat the second burger ? If not, that could be the burger indeed. 
By the way, I never tried this lens. Probably because I was always happy with the f/2.5 version.
Thanks for sharing,
Francis.
Main IQ differences between the two are the f/1.8 has virtually no field curvature or CA, where-as the f/2.5 has strong curvature in the corners and noticeable CA, which is worse when close. The f/1.8 only comes with straight blades, which is noticeable at f/2.8 when they are compared.
