For some reason, or combination of circumstances, the Bonnard exhibition at the Tate Gallery does not seem to have touched off as positive a reaction as might have been expected. Attendance has been quite high - on my own visit there I sometimes found it hard to see the paintings properly because of intervening heads and bodies; but the enthusiasm scarcely compares with what the Monet exhibition of a few years ago generated. Some people even claim to have found it all rather an anti-climax.

It is not, certainly, an ideal exhibition for the Tate, whose interlinked smallish rooms somehow seem wrong for the occasion, and the hanging and overall presentation are rather dull and functional. Perhaps, though, the true explanation lies at the very core of Bonnard's style, his intimisme; he is not really a public painter, and probably he is best appreciated by seeing a dozen or 20 works at a time, in a sympathetic setting where his subtleties can be fully absorbed. To an extent he is a painter's painter, whereas today's public expects something sensational or dramatic, or overtly autobiographical - as in Picasso, for instance. Nudes in baths, figures in interiors, still life, a sprinkling of self-portraits, lush gardens, a glimpse of landscape through an open window - superficially it is all fairly domestic and confined.

Bonnard was in fact a considerable innovator, but his subject matter remained largely traditional. It was probably this which made so many people during his lifetime consider him old-fashioned, particularly since there is scarcely a hint of the violence and anarchy of the times he lived through. He cultivated tranquillity and intimacy in a world which often seemed to have gone mad and dangerous, and inevitably, those who expect an artist to be always engage and politically aware saw this as a retreat into a self-created ivory tower. To a great extent his central obsession was to slow down time and rescue certain sights, scenes and moments from flux and oblivion.

He was also concerned - as is now generally recognised - with the actual act of seeing, and he often remarked sadly that the average person had no idea how to "look at things." Bonnard's optical register was exceptionally fine and his sense of focus is highly original, able to combine detail with a generalised vision as though he had taken in everything in a single glance. His ability to translate this into tones and colours was his own special secret, aided by probably the finest technique of any painter in the last hundred years - which is praise indeed when you think of such superb executants as Braque, Derain, Chagall and Beckmann. In spite of his domesticity, he knew how to command the Grand Style and to give a figure monumentality - but always related to its setting. His poses are sometimes very unorthodox and the figures are often glimpsed obliquely, either truncated ("cropped" as a photographer would say) or crowded into a corner of the composition. The nudes by now are relatively familiar, and he often packs them into a constricted, close-up space which adds to the emotional effect; on the other hand, he often places a reading, reclining or self-occupied figure with deceptive casualness in a spacious room or stretch of garden.

The various references to classical art have been tracked down by the Tate catalogue and show Bonnard to have been a learned painter as well as a supremely sensuous one. He knew himself to be an heir to a great tradition, and apart from the self-portraits he remains private and almost impersonal.

Bonnard faces both ways - backwards and forwards, with his roots in the late 19th century and his sights on our own time, or even beyond. Instead of starting with Cezanne, as most of the School of Paris did, his launching pad seems to have been the tonal tapestry of Gauguin and the major innovations contained in late Monet and Degas - particularly Degas's last pastels. Runs until May 17th.

It was always on the cards that Francis Bacon would pay posthumously for the rather feverish and uncritical praise heaped on him in his middle and later years. He took New York by storm, he was almost idolised in Paris (very rare for an English painter) and near the end of his life he enjoyed a triumph in Leningrad. Virtually every public gallery which could afford to buy him did so, and he was widely acclaimed as the greatest living painter, whose round face became as familiar through photographs as Picasso's sombre gaze and bald head.

Bacon the artist became almost submerged in his personal legend - the flaunted homosexuality, the drinking and drugging, the Soho pub-crawls and the semi-recognisable portraits of his friends and fellow-bohemians. Some of his encomiasts reached heights or depths of nonsense, particularly those who claimed that he was really a religious artist, the painter of "God's absence." The fading cliches of Existentialism were also dragged out to prove that he was the painter of le neant and the Void, expressing modern man's spiritual alienation and anxiety and psychological loneliness. (To be fair to the artist, he himself never made any such claims and in general avoided pseudo-metaphysics). Human nature, however, likes to tear down what it built up, so that the Hayward Gallery exhibition has been getting some stick, and an anti-Bacon tone seems to be surfacing in various places (about ten years or so ago, I noted a similar reaction against Ben Nicholson). It is called Francis Bacon: the Human Body and carries a eulogistic catalogue by Bacon's old buddy David Sylvester, who has curated the exhibition. Many or most of the pictures are familiar, which is no surprise considering the number of Bacon retrospectives there have been in recent decades, and the proliferation of books and articles about him. My own feeling is that they represent - to be blunt - a very mixed bag, and that Bacon was an exceptionally uneven painter who rose or sank in inspiration from one work to the next. His whole method of working could scarcely have produced anything very different from this shifting level. He was a gambler who cultivated chance and the "creative accident," he didn't make drawings (certainly I have never seen any) and preferred to "attack" the canvas in order to find his crucial image through the sheer potency of paint, he was a "bout" worker rather than a regular, disciplined one, and he probably relied too much on photographs to trigger off his imagination. Bacon was also, of course, a semi-alcoholic, like Augustus John, and alcohol and painting go badly together since artists need not only unclouded brains, but steady hands.

All this may explain why he was at times such a bad, slovenly technician, forcing the paint brutally and clumsily rather than making it obedient to his vision. It is also undeniable that he was rather a poor pictorial architect, relying on certain devices which he repeated a good deal. In the earlier work, that of the Fifties and early Sixties, he often enclosed his figures in a kind of spectral "box" like luminous wires. Later, he placed them in a circular space rather like a small stage set, often against a monochrome background - Henrietta Moraes, in one 1963 painting, sprawls naked on a bed with a purple-violet background and a black foreground. And in the big triptychs, he relied on tension and contrast rather than on constructive power.

Bacon was also an uncertain colourist, capable of achieving some utterly original effects alongside much tonal dullness and sheer bad taste. His paintings are, for the most part, miniature dramas or stage-settings, close to the Theatre of the Absurd and Artaud's Theatre of Cruelty, and they also exude an atmosphere close to film noir with its mood of big-city menace. In fact, his Paris influences are strong, including those of Giacometti and middle-period Picasso. Though a certain percentage of the Hayward show already looks dated or tired, and a number of works (including most of the triptychs) now seem inadequately realised, the originality and rather sinister spell of certain pictures are still hypnotic in their impact. Even now that their old shock element has largely evaporated, the Fifties paintings have a shadowy, tense, slightly menacing tonality and ambience which remain unique, while certain of the later works have a compelling power and originality. For example, the Figure in Movement from 1978, and the Oedipus and the Sphinx after Ingres, from 1983, must rank among the genuine masterpieces of their time; both stop you in your tracks. Bacon may sometimes give the effect of a man shooting in the dark, but he does so at a target which he alone could have envisaged. Runs until April 5th.

Elsewhere in the Hayward there is a very large exhibition of Henri Cartier-Bresson's photographs taken over many decades, to mark his 90th birthday this year. Many of them are familiar from books and reproductions, and they are so numerous that after a while I felt bemused and could look at no more. They cover a huge range of human types, activities, and events, with an omnivorous curiosity and zest, and an energy which seems to transcend a single lifetime. There are other Cartier-Bresson exhibitions running in London, of which the one at the National Portrait Gallery should not be ignored. This living Old Master seems to have been everywhere, and to have photographed everyone and everything. Runs until April 5th.