It’s the pity of it that hits you. The very last work in the National Portrait Gallery ’s new exhibition of Francis Bacon ’s portraits is like a gut punch. Triptych May-June 1973 is atypical of the artist’s work in that it clearly represents a real event, but then, not even Bacon’s imagination could compete with the horrible, prolonged death from an overdose of his lover George Dyer, in the bathroom of a Paris hotel in 1971.

The triptych, which depicts Dyer variously as he was found, slumped on the toilet; vomiting into a sink; and heaving with misery, almost engulfed by the inky darkness of his depression, is one of the saddest, angriest things I’ve seen – an anguished but controlled expression of the furious complexity of grief. Bacon considered portraiture the greatest genre of painting, for its ability to express what it means to be human. And like many artists, he was in his enthusiastic, not always constructive embrace of the good and bad of life, a sort of human 2.

0. The NPG show is the first major exhibition to focus on his portraits in nearly two decades. I was slightly dreading all those screaming heads and contorted bodies in one place – there are more than 50 works on display – but somehow, despite its darker moments, it is oddly uplifting.

It is organised by theme and, broadly, chronology, so it’s possible to see his painterly development from the screaming, boxed-in popes of the late Forties, and his decade-long flirtation with painting from .