Portraits of the royal family can tell us much about how their subjects' view them, argues Charles Saumarez Smith

George VI is having tea in the Royal Lodge, Windsor with his family. The King is sitting at his ease in a country suit with nicely polished brown brogues. Queen Elizabeth, as she then was, is about to pour a cup of tea. Her daughter, Princess Elizabeth, is leaning forward intently with Princess Margaret standing, about to take her seat at the table.

It is a perfect image of the postwar royal family - cosy, domestic, a slightly grander cup of tea than would have been available to their subjects but deliberately within the same orbit, and with a corgi asleep on the floor.

The painting, Conversation Piece at the Royal Lodge, Windsor by Sir James Gunn, which hangs at the National Portrait Gallery in London, represents the royal family at the high noon of its postwar popularity - the King and Queen as dutiful and conscientious servants of their people, paternalistic but accessible.

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Analysis of how portraits of 20th-century monarchs have been acquired by the National Portrait Gallery provides a snapshot of public attitudes to the monarchy; how they were perceived in terms of their public reputation and what was regarded as an appropriate form of iconography.

One might expect the ways in which the Queen has been represented to be low-key and conventional; but, in fact, the gallery has indulged in a form of late flowering monarchism which has mirrored the rise in the public visibility of the Royal Family, itself the result of greater media access to them in the past 30 years.

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The first portrait of the Queen to enter the gallery was the extraordinarily romanticised portrait of her by Pietro Annigoni, standing alone against a cloudless Hollywood sky - a highly theatrical, rather glossy image, creating perhaps a slightly false sense of the loneliness of the late 20th-century monarchy.

When it was unveiled in 1970, it was seen by 45,494 people in the first week and more than 200,000 in the first month. But it was controversial. The Evening News broke the story on February 25 1970: "Sorry, your Majesty. If this is what we, your subjects, have done to you in 15 years I can only apologise deeply... As Annigoni's chocolate box monarch of 28 you were gay, confident and full of life. In his new portrait all the joy seems to have gone. Those eyes, once sparkling, are worried and sad, with a tearful hint of a red rim."

The Times and Daily Telegraph too regarded it as inappropriate to comment upon a royal image negatively, Bernard Levin in the Daily Mail described the portrait as a "characterless smear of ugly pointless mediocrity", yet the public queued silently in multitudes to see it, with an attitude of protective deference.

The second image of the Queen acquired by the gallery shows her looking slightly uncertain and rather suburban, at home, in a yellow dress with her corgi. The artist, Michael Leonard, intended "a straightforward rather informal picture that would tend to play down the remoteness of Her Majesty's special position".

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The third image was a set of four screen prints by Andy Warhol - a rather more sophisticated image, ironic, detached, playing with the idea that the Queen now exists less in reality and more in reproduction.

Since I became director of the National Portrait Gallery in 1994, one of the things that has intrigued me is the way we display portraits of the contemporary royal family alone on the main staircase, on what is known as "The Royal Landing" in a slightly more sacred atmosphere than in other parts of the collection.

Like the debutantes who appear on the cover page of Country Life, they continue on the upstairs landing, not quite sure if they are a part of history or of contemporary life. They belong to a position that can neither be construed as separate and special nor as a ghetto.

In other words, the gallery replicates very precisely the position of the monarchy in British public life.

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Charles Saumarez Smith is director of the National Portrait Gallery.