In a layman sense we called that the black spot of M87* image is the event horizon. But in some technical references [1] the author said that the black spot actually delimits the photon sphere. I would like to know what is, precisely, that black spot on M87* image: Is the event Horizon or the Photon Sphere?
$$* * *$$ Feel free to be mathematically precise.
$[1]$ GRENZEBACH A. The Shadow of Black Holes.Springer.2016
