The Life and Lives of Dr Johnson, National Portrait Gallery, London

Litchfield's sage – larger than life

(National Portrait Gallery London)

Dr Johnson of Litchfield, the great 18th-century man of letters, was born exactly 300 hundred years ago this month, and the National Portrait Gallery is marking that centenary with an exhibition about the "Great Cham" (as he was so fondly known during his lifetime) in a slightly dreary room – the walls are of a sad, time-worn brown, and the 20 images are of uniformly modest dimensions and relatively colourless.

In fact, a person of small discernment would pass this room by altogether because, at a superficial glance, it seems to promise so little by way of visual spectacle. That would be unwise because these 20 images – a suite of miscellaneous engravings and drawings, most done either during Johnson's life-time or soon after – tell the tale of a fascinating man who made of his own life and manifold eccentricities a truly absorbing sideshow.

So here are images of some of the people amongst whom Johnson lived, and upon whom he made his mark. First up is the great actor David Garrick, a life-long friend. Before he took himself off to London, Dr Johnson ran a school in Litchfield, and Garrick was one his pupils. One of very few. The school soon died for lack of interest. They travelled to London together to re-make themselves as metropolitan celebrities. Both succeeded admirably.

In fact, one of the faults of this quietly-spoken exhibition is that, because these images, generally speaking, were made to show their subjects in their best possible light – enhancing Mrs Thrale's beauty, for example, in a wonderfully sensitive profile drawing by George Dance, regularising the features of James Boswell, Johnson's indefatigable biographer – we don't really understand just how odd Johnson was as a human being.

In order to do that, we need to read what Boswell and so many others said about him – his tremendous physical ungainliness, his oddly repulsive eating habits and violent squints. The images of Johnson we see on these walls have, for the most part, smoothed all those rich human imperfections away.

What we are left with are the sage; the indefatigable master raconteur; the great amateur scholar poet; the pioneer of the making of modern biography; the essayist and dictionary-maker; the man who, like all good journalists, wrote at speed, in the full flush of his teeming brain and, if necessary, corrected later – fingering his pen, leaning world-wearily on his books, looking into the far distance in pursuit of yet another marvellously vituperative sally against the Scots, or some other, equally benighted, race.

Yes, that was another thing that he gave to journalism, the sheer, forceful quality of his opinions. Why be bland when you can offend so magnificently, with such unforgettable turns of phrase?

He became very famous for being himself – for not deviating from the marvellously abrasive Tory curmudgeonliness that he cultivated so assiduously, to the delight and mock-indignation of so many. And the public loved him for it. And he loved them for loving him, and encouraged them – and his slavishly devoted biographer Boswell especially – to take note of what he said and did, so that he would in time become the stuff of legend.

But legends can get out of hand. Two of these images show him frowning and scowling and sneering from beyond the grave – in one he is a portrait bust on a plinth, looking down on a trio of idle biographers, and in the other he hovers like a vengeful ghost, and tries to dissuade his generous friend Mrs Thrale from inventing correspondence between them. These images seem to be saying: "Why make it up? Was I not sufficiently extraordinary anyway?"

To 13 December (020-7306 0055)