Caracalla (Pagan, Crisis of the Third Century, ~210 AD)
Decius (Pagan, Crisis of the Third Century, ~250 AD - note that, despite being only ~35 years away from Diocletian, the facial features here are very rough and ‘realistic’)
Diocletian (Pagan, Late Empire, ~284 AD - note the departure in style - smooth face, large eyes)
Constantine (Christian, Late Empire, ~320 AD - very idealized, almost wiped clean of features)
Valentinian III (Christian, Late Empire, ~455 AD - idealized into abstractness, the ‘idea’ of the man who is Emperor)
Oh, and one for the road - a portrait of a NON-Emperor, 5th century AD - note that though the style has hints of the Christian Emperors, it has far ‘rougher’ (and IMO, more beautiful) detailed features - because it’s not trying to be the ‘idea’ of the Divinely Ordained Leader. It wasn’t a loss of talent (that doesn’t come 'til later, when the entire Empire collapses in on itself) - it was a weird stylistic choice.
For more comparisons:
Vespasian (Pagan, Principate era, ~70 AD)
Caracalla (Pagan, Crisis of the Third Century, ~210 AD)
Decius (Pagan, Crisis of the Third Century, ~250 AD - note that, despite being only ~35 years away from Diocletian, the facial features here are very rough and ‘realistic’)
Diocletian (Pagan, Late Empire, ~284 AD - note the departure in style - smooth face, large eyes)
Constantine (Christian, Late Empire, ~320 AD - very idealized, almost wiped clean of features)
Valentinian III (Christian, Late Empire, ~455 AD - idealized into abstractness, the ‘idea’ of the man who is Emperor)
Oh, and one for the road - a portrait of a NON-Emperor, 5th century AD - note that though the style has hints of the Christian Emperors, it has far ‘rougher’ (and IMO, more beautiful) detailed features - because it’s not trying to be the ‘idea’ of the Divinely Ordained Leader. It wasn’t a loss of talent (that doesn’t come 'til later, when the entire Empire collapses in on itself) - it was a weird stylistic choice.
Bonus bonus points - you can see the same thing happen in depictions on coinage.
Coin depicting Vespasian (Pagan, Principate era, ~70 AD)
Coin depicting Caracalla (Pagan, Crisis of the Third Century, ~210 AD)
Coin depicting Constantine I (Christian, Late Empire, ~320 AD)
Coin depicting Julian the Apostate (Pagan, Late Empire, ~363 AD)
Coin depicting Valentinian III (Christian, Late Empire, ~455 AD)