On the morning of October 10, 1940, a photograph taken by Fred Morley of Fox Photos was published in a London newspaper. It showed a milkman cheerily delivering a dozen bottles of milk to Londoners, an extra bottle stuffed in his jacket pocket. This was clearly no ordinary day’s work, however, because the milkman was making his way across a scene of devastation. This was the morning after a German air raid. Walking gamely over the rubble that filled the frame, the milkman was a symbol of British pluck and determination. This was the 32nd consecutive day of German bombing raids, and yet life went on.

What readers did not know was that the milkman was actually Fred Morley’s assistant, persuaded to dress up as a milkman and walk chirpily through the rubble towards the camera. Censors at the Ministry of Information would have suppressed images of large-scale destruction for fear of eroding morale, but Morley got round this with a little subterfuge.

The picture is one of the high-lights of an exhibition of press photography at the National Portrait Gallery looking at developments in the medium during the Fleet Street era, from 1904, when the Daily Mirror became the first daily newspaper to carry photographs, to 1986, when the Fleet Street era ended with the exodus of newspapers to other sites.

Press photography and photo-journalism live with some ease in the present museum world. As long ago as 1943 Weegee’s famous image The Critic, of two society ladies ablaze with diamonds entering the opera oblivious to the angry comments of a member of a lower class, was exhibited at the Museum of Modern Art in New York. (Weegee too had doctored the situation by plying “the critic” with drink and escorting her to the scene.)

In the past ten years handsomely designed exhibitions of press photography have become so common that many news photographers now shoot with future art-gallery or museum fame in mind. The problem with this exhibition is that, between the dates it covers, news photographers were shooting for newspapers or news magazines and nothing else. They never once thought they were making “art”. Their simplifications and heightened contrasts were consciously designed to make their pictures carry well in half-tone reproduction. Each picture was like a piece of toast. Eat it while it’s hot.

The National Portrait Gallery’s parade is not mere news, but true newspaper material, in dark tones of black and grey, with an insistently hard stare. It makes for a dark grey show. You will have to like newspapers for their own intrinsic qualities to really like this show.

Historically, it is interesting that there has been a consistent focus on much the same subjects throughout the period, with most attention paid then as now to politics, royals, sport and society. The one blessed relief is the absence of the kinds of flaky celebrities whose faces fill our newspaper pages today. But signs of what was to come is seen in one celebrity shot, a quaint tableau photographed by Jack Esten in which the actress Merle Oberon is giving a press conference at the Savoy Hotel in 1956. She sips tea elegantly from porcelain while photographers in suits and ties clamber around her suite with their cameras and flashguns, bulky bags and snaking cables.

Of course, royalty has always been the subject of enormous press scrutiny, but we are shown in the images selected here how the photographic treatment of royalty at the time of Edward VIII’s abdication changed in line with shifts in public opinion.

During the 1920s, while the Duke of York, the future King George VI, was being given a benign image, for example, pictured awkwardly clinging onto a fairground carousel horse, his older brother, Edward, Prince of Wales, was building a reputation as a debonair and worldly heir to the throne. The press, as Roger Hargreaves writes in the book that accompanies this exhibition, happily colluded in reinforcing the stereotypes, with a public image for Edward of movie-star proportions.

News of his affair with the American divorcée Wallis Simpson had been suppressed by the British press, but when public opinion swung in favour of abdication, his fall from grace was underlined by the dramatic change in his treatment in the press. Gone were the matinee-idol shots, replaced by flashlit pictures of the disgraced King slumped in the back of a car as he was driven away from Windsor Castle. Suddenly photographers were working at point-blank range the better to catch him in the raw.

The press cameras were beginning to fasten tenaciously on to the common fascination with royalty, and we still stare raptly at the results. The last image in the show is of a 19-year-old Lady Diana Spencer, a part-time assistant at a Pimlico kindergarten, being heckled by a photographer and turning to face his lens in the street. She was ill-prepared for the media onslaught that accompanied the news of her relationship with the Prince of Wales but she soon learnt how to play the media.

Ending the show at this point feels odd and disjointed. The Fleet Street parameters are tightly drawn. So much more in the world of press photography was to be unleashed with Diana, but we get none of that.

Picture editors and press photographers will enjoy this show, but for the rest of us it would have been better left as a book.

Daily Encounters: Photographs from Fleet Street is at the National Portrait Gallery, London WC2 (www.npg.org.uk), from Thursday

Editor's choice

• The 100 Best Films of 2009
• The 100 Worst Movies of 2009
• The 100 Best Films of the Decade