Arts review: Laura Knight Portraits, National Portrait Gallery, London

'Modern' is not a word you automatically associate with Dame Laura Knight, but her self-portrait is just that, in some ways at least. Note I say "modern" and not "avant garde": the picture dates from 1913, the year Picasso painted Guitar and Duchamp made his wheel-stool, Roue de Bicyclette.

Knight is having no truck with that nonsense. She has clearly been to see Roger Fry's Second Post-Impressionist Exhibition, but just as clearly been picky about what she found there. A little light Fauvism, yes. Cézanne and Cubism, no. But it is not her self-portrait's style that makes it “modern”, it is its composition.

The picture's vanishing point, oddly, lies on the fold of a screen, drawing our eyes back to nothing very much. It is what happens to either side of that fold that grabs our attention. To the left, is Knight herself, her head in full profile like a queen on a coin. To the right, on the same level, is a bottom - rather a pink bottom, it has to be said, as though freshly slapped. This belongs to Knight's friend the enamellist, Ella Naper, model and neighbour at Lamorna Cornwall.

The two women had collaborated on a series of ballet works, and here they collaborate again, acting out a drama from which we are pictorially excluded. There are two faces in the portrait and yet we only see half of one of them, and that expressionless. Neither woman looks at the other, both are shown back view. At the composition's heart is a blank.

It is a charming picture, in large part because it doesn't set out to charm. You can look at it if you like, but it's up to you. That is not how things are meant to be in art made by women. The critic John Berger summed it up when he wrote that men in paintings act, women appear: “Men look at women,” Berger said. “Women watch themselves being looked at.” This was particularly so if they had no clothes on. In a reversal of natural order, neither Knight nor Naper gives a toss whether you look at them, dressed or nude. They don't even know you exist.

In its reasonable, English way, the work is a manifesto painting. In 1936, Knight would become the first woman Royal Academician since Angelica Kauffman. The seeds of that ambition are there in her self-portrait, but so, maybe, is the problem with her art. She can be a great painter, but only on her own terms.

The loveliest images in this show - and there are a number - are the ones Knight made off her own bat. The cheesiest - and there are a number of those as well - are works she made on commission. Notable in the first group are the pictures the soon-to-be Dame Laura painted in Baltimore in 1927. While her husband, Harold Knight, made portraits of the surgeons at Johns Hopkins Hospital, Laura Knight set up her easel in its racially segregated maternity ward. If her description of the women there as “darkies” rings unhappily in modern ears - she was born in 1877, after all - there is no questioning the empathy she felt for them.

Pearl Johnson, in watercolour and pastel, is in part an exercise in the effects of light on dark skin, but also a study in trust. Implicit in the power of Johnson's gaze is the knowledge that it is directed at Knight, not at us. The same is true of Knight's paintings of gypsy women at Epsom, likewise made quickly, en plein air and in watery oils that feel informal and intimate.

By way of absolute contrast is the best-known image in the National Portrait Gallery's show (and perhaps Knight's most famous painting overall), Ruby Loftus Screwing a Breech Ring. You'd have thought that its subject - a woman doing a man's job - would have been right up Knight's street. Loftus was an ordnance mechanic, her portrait commissioned by the War Artists Advisory Committee to encourage other women into munitions work. The Soviet Union had Alexey Stakhanov and we had Ruby Loftus, a hair-netted heroine shown in a style midway between Socialist Realism and the Saturday Evening Post.

And her portrait is dreadful. Compared with the work of Knight's most obvious competitor, Norman Rockwell, its composition is clumsy and brushwork inept. Ruby Loftus is a skill-less depiction of skill, the rough stippling of its left shoulder horribly at odds with the Ingres-like burnish of its machine tools, Loftus's full-profile depiction is a sad reprise of Knight's self-portrait of 30 years before. Perhaps Dame Laura mistrusted propaganda, or perhaps she didn't like being told what to do by the WAAC's male director, Kenneth Clark. Either way, the outcome is woeful.

National Portrait Gallery; to 13 Oct