However successful photographers may be in capturing the world and life, there is something in them that seems to feel that portraiture is the higher form in which their craft challenges art, just as the portrait painters of the 17th and 18th century looked up to religious art as a higher form.
"The personality of living people, dual and multi-fold, is always more absorbing than that portrayed on canvas," wrote E O Hoppé (1878-1972), whose work is on display at the National Portrait Gallery at the moment, "and I have been lucky in that my calling as a portraitist has enabled me to peek behind the faces, as it were, of so many great and interesting men and women."
It was a sentiment that Ida Kar (1908-74), whose portraits now join the Hoppé show, would have profoundly agreed with. A younger contemporary of Hoppé, she took her studies of artists as, in their way, measuring up to them.
The two exhibitions make a fascinating contrast. Both Hoppé and Kar came as immigrants from abroad. Hoppé arrived early in the century as the scion of a German banking family seeking a more creative life in London. Kar came as the Russian-born Armenian wife of an English art dealer she met in the Second World War in Egypt. Both worked for magazines, producing regular series of portraits of the great and good; Kar in the art world, Hoppé more of stars of the performing arts.
But it is their differences that make the most interesting juxtaposition. Hoppé used artificial lighting and shallow focus to concentre on the face and to give his subjects an aura of significance. Kar preferred natural lighting and to photograph her subjects in their work setting, the artist studio or the writer's study. It's the choice that still determines much of modern photographic portraiture: to go in tight or to pull out to give the context – Irving Penn versus Richard Avedon.
Kar's approach produced a formidable portrait series of artists working in Britain and Paris in the post-war years. There is Stanley Spencer, under the umbrella he always carried with him, his shirt undone, his suits baggy and his look quizzical. Henry Moore is set at the end of a row of sculptures as if he was one himself; Gino Severini leans on the table with the still life he was painting, his hat made from the daily newspaper firmly affixed on his head.
Her work is better with artists than writers partly because a backdrop of books is less meaningful than a surrounding of artistic output. But she also seems to me to respond more to men than women. Barbara Hepworth behind a wire construction is clever, but tells little of the artist behind.
It's quite different with her men. A study of a shirtless Hussein Shariffe is blatantly erotic, as are the smouldering eyes of the Trinidadian writer Samuel Selvon. But it was really with older men that Kar achieved her best work. She was clearly devoted to her father, and kept a portrait of him always with her. And that sympathy for the grizzled face of experience shows. There are truly stunning portraits of Jacob Epstein, the hands of a worker and the face of a Lower East Side New Yorker; Augustus John, an angry old man hungry for new life; and Marc Chagall, the hand crooked and eyes full of regret.
Hoppé is quite different. He loved women. The few nude portraits on display are all too aware of the attractions of the flesh and his pictures of performers, unlike Kar's, understand absolutely the face that women put on to the world on the stage and on the screen. The mesmeric stare of the Austrian-born dancer and actress (later Countess of Carnarvon) Tilly Losch justly provides the cover of the catalogue. But most of his portraits are much more gentle and affectionate. Margot Fonteyn, at 16 in 1935, is a sweet image of pulchritude, a world away from the angular elegance of her later years. There is a moving picture of Ellen Terry, betraying the sadness of her part (the Nurse in Romeo and Juliet) but also the regret of her own advancing years.
His best pictures of men are in a way those who are most feminine, or most keen on face: Alan Odle, the English illustrator, attempts to seduce the camera, Ezra Pound stares imperiously down on it, Nijinsky looks at his most androgynous. With The King's Speech in mind, the portrait of George VI and his wife, when still Duke and Duchess of York, is touching in his shyness and her uncertainty, although it has to be said that the more formal portraits of his frightful father, George V, and the great and the good such as Kipling, Conan Doyle and Somerset Maugham are staged but unrevealing.
Which is the problem with photographic portraits. The ambition of many of its practitioners is that the camera can catch, in the way that they believe paint cannot, the inner soul of the subject. But the very act of staging a shoot, in the studio or in the home, and the composition of the shot is an act of self-consciousness by the sitter and courtship by the photographer. It works well with performers because theirs is a self-conscious art. But what does one say of the picture of Benito Mussolini in 1924? The lighting and the look inevitably heroicise the politician rather than penetrate the man behind the macho stare.
Teasingly, the National Portrait Gallery tags Ida Kar with the title "Bohemian Photographer". Well she may have photographed the so-called "bohemian" set in Soho at the time, but bohemian she definitely wasn't. She was invited to Cuba and to her homeland of Armenia, where she was treated as a national hero. Her pictures of the local scenes there are too ordered to be truly compelling. It is only with a final venture taking nudes – served in this exhibition by an extraordinary image of a naked mother and child of 1974, whose representation of a nude young girl caused controversy then and would do even more now – that one begins to feel that Kar could have developed from portraiture to something broader.
Not so with Hoppé. From the start he seems to have felt that there was something wrong in just portraying the well-known and the aspiring. In the manner of August Sander he tried photographing "types" rather than individuals, as in his books Taken from Life (1922), London Types (1926) and The Book of Fair Women (1922), where, in homage to his favourite subject, he took "Beauties" from around the world to show the variety of feminine attractiveness. It doesn't really work, successful though some of the individual pictures are. The very idea of types and examples deprives the camera of its greatest assets in portraiture, which is concentration on the individual.
Hoppé found his true calling in his street scenes. Taking his camera, sometimes secreted, onto the street, he took a series of pictures of life at its most human and humorous. Wit is a rare quality in photography (Jane Bown is unusual in contemporary work) but Hoppé had it in full. His pre-war pictures of Savoy Hotel waiters feeding birds, the sizable rears of men and women exiting Tottenham Court Road tube station and of boys doing stretches at King's School, Canterbury are genuinely hilarious. Proof that, in photography, actuality always did have the edge on portraiture.
Hoppé Portraits: Society, Studio and Street, National Portrait Gallery, London WC2 (020 7306 0055) to 30 May; Ida Kar: Bohemian Photographer 1908-1974, National Portrait Gallery, to 19 JuneReuse content