For such a small country, Cuba is big on iconic images. Some of the most famous photographs from the second half of the 20th century record the triumph of the Cuban Revolution, which took place 50 years ago on 1 January. The momentous events, which inspired enormous devotion around the world and great fear in Washington, are marked by an exhibition by the photographic co-operative Magnum Photos, "Cuba: 50 Years of Revolution".

The exhibition includes some of the best-loved photos of the Cuban revolutionaries, such as René Burri's 1963 shot of Che Guevara in his office smoking a cigar, which has been widely reproduced, with and without permission. What makes an image such as this iconic? It is too easy to say - although there is a certain truth in it - that the truly memorable images of Cuba belong to the heroic period of the revolution, which did not outlive the demise of Guevara. I am inclined to follow the analysis by Robert Hariman and John Louis Lucaites in their recent book, No Caption Needed, in which they argue that the iconic photo, with a few exceptions, is a dramatic enactment of a politically emotive scenario. Like the raising of the US flag at Iwo Jima, it is an image with more than documentary value that exemplifies ideology at work, and enters and remains in circulation because it activates collective memory.

This goes a long way to explaining why Burri's photograph of Guevara - like that other photo, by the Cuban photographer Alberto Korda, in which, as Richard Gott has felicitously described it, Guevara gazes "fiercely into some distant horizon" - became one of the iconic images of the times. Neither photo fixes a specific historical moment, but rather they evoke a whole ethos; and they affect the viewer according to the ideological sentiments the photograph suggests. (These may be deeply buried - now adays many of those who wear Che Guevara T-shirts hardly know who he was.)

The problem for the curators of the Magnum exhibition is that the handful of truly great images inevitably overshadows all the rest. There is a lovely photo from 1954 by Eve Arnold - the earliest in the exhibition - of a fisherman with his wife and child, which was included in the "Family of Man" exhibition at the Museum of Modern Art in New York in 1955. It appears here beside two other Arnold photos - a pair of dancers at the Tropicana nightclub and a "bar girl in a brothel" (which looks like a still from a Latin American melodrama of the period, except that the girl here is of mixed race, a mulata). This image does not attain the classic status of the Burri, as it is abstracted both from history and place: the fisherman's family does not speak specifically of Cuba, but could just as easily have been taken in Mexico or Colombia.

Other images draw upon figures and objects which have come to represent Cuba in the col lective imagination of the outside world. David Alan Harvey's terrific 1998 picture captures a folkloric ballet troupe rehearsing in a courtyard, and Christopher Anderson's 2003 street scene features a classic 1950s motor car, of the kind that adorns the cover of many a tourist guidebook. Both are highly characteristic Cuban scenes, but while Harvey's dancers are suspended in dynamic poise, Anderson's automobile, captured on a cheap Holga camera, ineluctably fixes an object whose iconic significance is in fact quite changeable. These cars have shifted their symbolic meaning: back in the 1950s they signified Cuba's modernity; then, as the United States turned its back and the island fell under Soviet tutelage, they came to signify its arrested development. Nowadays they have become trophies of postmodernist retro, sought after by foreign tourists prepared to pay hard dollars for them.

One or two of the images in the exhibition are almost anti-iconic. Take Burt Glinn's photo of Fidel Castro riding into Havana a few days after the victory of the guerrillas. It is a moment of high drama, but it is those closer to the lens, not Castro, who draw the eye. More problematic is Andrew Saint George's shot of Castro speaking at a rally in 1960, in front of a battery of microphones, face upturned, his arm raised and finger pointing upwards in a characteristic gesture. This is not, I dare say, an image that his followers would accept as iconic, because it is equivalent to the commentary in an American documentary of the same year, Yanki No!, shot by Richard Leacock and Albert Maysles, which introduces a scene with the line, "Fidel Castro, who looks like a raving madman to North Americans, is seen by Latin Americans as a sort of messiah." It is no surprise to learn that, unlike Burri, who frequently returned to Cuba, Saint George was soon unwelcome on the island.

The trouble is that foreign photographers are, almost by definition, tourists attracted by Cuba’s façade

There is only a smattering of photos from the 1970s and 1980s, when Cuba was relatively closed to visitors who were not Latin American, and from the 1990s the character of the images changes completely. In part, this is the result of Magnum's own shift away from an earlier focus on photojournalism, towards the terrain of art photography; this is enough to explain the character of the images by Martin Parr and Miguel Rio Branco, for instance, the former being rather bland, the latter almost completely abstract. Both compare poorly with Cuban art photography by the likes of José A Figueroa, Rogelio ópez Marín and José Manuel Fors. What they lack is what the poet Eliseo Alberto described when he said that the secret of his friend Figueroa's images "is not in what can be seen but in what is suggested, in what is hidden and not only what is shown".

This is characteristic not only of Figueroa. Cuban photography moved away from the heroic subjects of the early years as there emerged a generation who were still children when the revolution occurred. This generation turned towards the celebration of "everyday heroes", the common man and woman who had not been photographic subjects before but were now celebrated in new photo magazines and exhibition spaces. Here they were seen as agents of social transformation - for example, in a magnificent series of pictures of cane cutters by Enrique de la Uz - but with a graphic sensibility that was a long way from Soviet-style socialist realism, which never appealed to Cuban artists of any kind.

Then, by the 1980s and with the emergence of a third generation, born after the revolution and with no memory of their homeland before Castro, Cuban photography began to align itself with the avant-garde and often adopted multimedia and installation formats. Official response vacillated, but Cuban photography was now firmly committed to a sense of irony and metaphor that questioned prevailing conditions without falling on the wrong side of Castro's famed formulation back in 1961: "Within the revolution, everything; against it, nothing."

The German poet and essayist Hans Magnus Enzensberger wrote, in an essay called "Tourists of the Revolution", about the way that socialism became a secretive affair, accessible only to those with the opportunity to peek behind the mystifying façade. The trouble is that foreign photographers are almost by definition tourists attracted by the façade - in which, to be sure, the best are able to discover an arrangement of features that appears to reveal a deeper truth. All too often, however, they remain outsiders.

In the end, an exhibition like this tells us less about Cuba itself than about the susceptibilities of outside observers, as seen through the eyes of the curator. If the curator has no first-hand knowledge of the subject, the selection becomes arbitrary - whereas Harvey's portfolio of 114 photographs of Cuba in 1998, available for viewing on the Magnum website, gives a much better idea of what the country is like.

The photos are displayed on the wall without captions, which are to be found on an accompanying sheet. These, I am told, were written by the photographers, and again betray their position as outsiders: they are often simply too literal to be enlightening. In one case the caption is misleading and the photo a mystification. This is the Iranian photographer Abbas's picture of 1994 captioned "Passengers perched on an impossibly small raft leave Cuba behind and set out for the USA". The photo, which shows Havana in the background and a number of figures in the water swimming away from the few left on the raft, needs deciphering. It is obviously taken from another boat, probably manned by coastguards, and the swimmers must be trying to escape back to land. Many of the balseros, or rafters, who got past the coastguard died watery deaths before they ever arrived in Miami.

The year before Abbas's shot, Figueroa took a photograph looking down on the parapet of the Malecón, Havana's seafront promenade. On the seaward side, battered by the ocean, stands what looks like a line of crosses. The photo is entitled simply Homenaje ("homage") - a perfect example of the poignancy of suggestion, and one from which many of the photographers in the Magnum exhibition could learn.

"Cuba: 50 Years of Revolution" is at the Magnum Print Room, London EC1, until 30 January 2009. http://www.magnumphotos.com

Michael Chanan is a film-maker and author of "The Cuban Image" (BFI Publishing)