The concentration of excellence in this exhibition is breathtaking. It’s notionally about Leonardo’s period in Milan, between his early training in Florence and his later itinerant years in Florence and elsewhere. But Leonardo’s work is so familiar that no one expects new insights into his work; this is a display of masterpieces in context, paintings and drawings by Leonardo shown alongside works of his close followers. Here we see both versions of the Virgin of the Rocks, and the recently attributed Salvator Mundi, together with many of his other most significant artistic achievements.

We see Leonardo constantly striving to depict the world more perfectly, by doing things noone had thought possible. Some of the drawings show his fecund imagination in overdrive: he drew and re-drew the same composition, sometimes side by side, sometimes one on top of another. But while Leonardo is well known as scientist and as draftsman, here above all we see him as painter. The first great masterpiece in the exhibition is the portrait of Cecilia Gallerani, the lady fronting the publicity campaign. Leonardo takes forward the Netherlandish innovations of oil painting and three quarter view portraiture, but better than anyone before he captures a sense of volume through precise depiction of the transitions between light and shade.  Most striking is her firm stare, looking at something outside the picture frame, but never focusing on the viewer.

While much of the painting is well preserved, the entire background has been re-painted, as have the fingers. A number of visitors have mistaken the later inscription for a signature, and some critics have even seen the restored fingers as particular highlights! It would be helpful if the wall text spoke to this point. The contrast with the famous Belle Ferroniere from the Louvre (in the same room) is interesting. The condition is rather compromised, with disfiguring repaint on the nose and jaw. This figure also stares at something outside the picture frame, avoiding our gaze. But the depiction of volume is less successful than the Cecilia, particularly when viewed from the side.

Keep these paintings in mind when you go upstairs to the permanent collection, and compare these portraits with those of Rembrandt and Velazquez. These later baroque masters took a very different approach; their sitters fix their eyes upon the viewer, drawing you in. Rembrandt’s great late portraits seem to be looking right at you from any vantage point, an astonishing achievement very different from Leonardo’s. In their way, these later paintings show an even more thoroughgoing and engaged ‘humanism’ than Leonardo’s more distant visages. Our understanding of Leonardo is coloured by stories of universal genius and renaissance humanism that sometimes obscure his individual contribution and cause us to misunderstand later developments.

The Madonna Litta from the Hermitage is not generally accepted as a Leonardo, and seeing it among its peers makes it very clear why. It’s painted in tempera rather than Leonardo’s normal oil and is damaged in places (particularly the blue robe), but even the better preserved areas are oddly handled and do not show Leonardo’s touch. The Christ Child in particular is simply not good enough to be his. I don’t believe for a moment that the exhibition curator considers this to be by Leonardo; the catalogue entry is written by a Hermitage propagandist. This is a really corrupting move that undermines the National Gallery’s credibility and makes a mockery of its scholarly credentials. Lenders should not be able to dictate how their loans will be displayed and attributed.

Some of the paintings by Leonardo’s followers are really very good, although all suffer from proximity to genius. Boltraffio was very talented, particularly as a draftsman. His Artemisia is very damaged (although condition is not mentioned in the catalogue), but in conception and in the less damaged passages it excels. Marco d’Oggiono is weaker (although The Archinto Portrait is worth seeing), as is the Master of the Pala Sforzesca.

The catalogue defends the National Gallery’s claim that its version of the Virgin of the Rocks is indeed by Leonardo, whereas it has generally been regarded as in large part completed by assistants. Comparison with the Louvre’s version does not flatter. The landscape is flatter, parts of the figures are cursory. The parlour game of attributing different elements to assistants can never be concluded; it may be by a bored Leonardo, or parts may have been filled in by assistants. I favour the latter theory, but what’s certain is that it’s a much weaker painting than the Louvre’s masterpiece, despite the latter being rather damaged in places. The National Gallery’s excellent technical bulletin has generously been provided free on the internet and provides useful background on these paintings, and more generally on Leonardo’s technique.

The Salvator Mundi is a horribly damaged ghost, picked clean of overpainting in a two-year restoration. I came to this painting as a sceptic; it was just too convenient for curators to have ‘discovered’ a Leonardo just in time for the exhibition. But I now think what remains is by Leonardo – the blessing hand and parts of the hair suggest the hand of the master. Nevertheless, circumspection is appropriate given the degree of damage. It’s a bit like trying to attribute based on a bad photograph. It is in any case unseemly for the National Gallery to lend its authority to the attribution of a privately-owned painting. I also agree that the Madonna of the Yarnwinder is largely by Leonardo, albeit with a weak landscape background completed by another hand. I saw this fine painting on loan in Edinburgh recently; it will be well worth a trip to see it if it returns.

The final room is devoted to the Last Supper, the ruined fresco in Milan. A near full scale copy is supplemented with preparatory drawings, including some of the greatest drawings in the whole history of art. The study for St Philip is just outstanding. On the other hand, the head study identified as Leonardo ‘with additions’ looks actually to be a rather slavish copy, with firm contours and a less successful treatment of volume than is seen in the master’s own.

Despite the greatness of the art in this show, the adulation of critics is misplaced. Praise due to the artist has been bestowed on the curator. The National Gallery is a powerful cultural institution that has obtained loans that no responsible owner would have permitted. There were quid pro quo deals that will see fragile artworks from the National Gallery itself lent abroad. And the much-travelled Cecilia Gallerani was even rented for the exhibition, against the National Gallery’s own policy of never paying lenders (evaded by getting an investment bank to pay as part of a sponsorship deal). They are then displayed in conditions so crowded that little can be taken in. It has taken me seven visits to write this review, which was possible only because I got a season ticket.

Some people seem to confuse art exhibitions with rock concerts, seeing the crowd as part of the experience. But surely the point of an art exhibition is actually to see the art; galleries are not just another space for hanging out with fellow humans. There are at most nine paintings by Leonardo in this show, and every half hour, 180 people are allowed in - suggesting about twenty looking at each painting at any given time. But if everyone allows just ten minutes per painting, as the curator suggests, and spends some time with the other exhibits, there will be scores of people in front of everything. If people stay for two and a half hours, that implies about 900 people at any one time - say, 50 people per Leonardo painting and five or six in front of each drawing or painting by a follower. In practice, of course, the gallery relies on many people racing through and taking in little - which is an understandable response to the sheer pressure of numbers. What really rankles is the nasty cynicism of the press campaign that boasts that they’ve allowed in fewer than the absolute maximum allowed under health and safety rules.

Anyone with a serious interest in art will go anyway; that goes without saying. But it’s sad to have to view these works under such atrocious conditions.

Till 5 February 2012