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Who, what, where? Magnum is hoping the modern crowd will help them identify a historic archive

The photo agency has thousands of shots that lack proper captions

Given the choice between killing time in front of the TV, or playing an online game, which would you do? Judging by their next venture, the bigwigs at Magnum are hoping you opt for the latter.

The co-operative picture agency is recruiting photo enthusiasts to help it "tag" – or identify – some 200,000 images from its archives.

Put like that, the project doesn't sound too special. Crowd-sourcing is a well-established technique for accumulating data; think Wikipedia, with its collective, DIY ethos. But this isn't crowd-sourcing as we know it. Instead, an added element has been introduced: gaming. Using a series of incentives, from leader board-style status-enhancers to virtual rewards, Magnum hopes to make the tagging process more fun, and, as a result, more popular.

Tasks range from identification (labelling a picture of the Eiffel Tower, and contributing some information) to contextualisation (specifying that you mean Paris, France as opposed to Paris, Texas), to moderation, fact-checking and verification.

In return, says Todd Carter, co-founder and chief executive of Tagasauris, the photo-tagging company collaborating with Magnum, those doing the tagging get privileged access to pictures, fun, and a sense of participation, as they compete against one another. "The idea is to take a task but to make it enjoyable, and to connect it socially, whether that's by introducing actual rewards or the potential to gain in status," he explains.

Social networking is another key part of the strategy. Tagasauris already registers some 2,500 taggers working across the Magnum archive. There is, however, one problem with their work: it isn't necessarily specialised. While these taggers might be able to offer general information, for the most part they lack the kind of expert knowledge that could contribute more meaningfully – the nature of the film used for a particular shot, perhaps, or the back story of the photographer.

Magnum hopes to recruit would-be gamers with superior expertise through social networking – it currently has more than 300,000 followers on Twitter and 135,000 Facebookers who "like" its page. Every time there is a new image to be inspected, it will be posted on Magnum's Twitter feed. Participants can share the photos, spreading the knowledge base wider. "It's similar to the idea of a Twitter hashtag. If you're watching the Oscars, and you want the opinions of people who know about these things, you can follow the Twitter hashtag," says Carter.

Recent years have seen an explosion in similar projects – twists on the crowd-sourcing concept that have harnessed expertise. At Oxford University, the Galaxy Zoo project has seen more than 250,000 people participate in refining a database of galactic images. The American Library of Congress called on the public's historical expertise to help it identify stored documents and pictures. Such projects, says Dr Eric T. Meyer, research fellow at the Oxford Internet Institute, tap into an innate desire to be part of something bigger: "People like the feeling of contributing – particularly if it's in a field that they have an interest in."

Carter insists there's yet more to be gained by having contributors compete with one another. When The New York Times photojournalism blog posted an article on the Magnum project, several thousand people contacted the agency. "The question is: how much broader would that response have been if the task was gamified? My suspicion is that there are millions of eligible people who would do it," says Carter.

Americans and Europeans, he reasons, spend billion of hours a year watching TV – and are increasingly moving online to do so. All that leisure time adds up to a lot of photos tagged. The popularity of other social-networking games such as Farmville and Mafia Wars demonstrates how compelling online gaming can be.

At the moment, the project is limited to just 50 volunteers – although if this early pilot works, that number will expand indefinitely. It is, of course, bad news for those used to making a living from similar tasks; critics point out that crowd-sourcing relies on free labour.

Given Magnum's co-operative credentials, it may not quite be slaving for The Man, but it's still different from volunteering to help a museum. It's a problem Carter hopes people will see past: "If you look at famous events in history, there's a Magnum photo to document it, but so many aren't visible because they're not tagged. The only companies that could afford this kind of tagging are giants like Microsoft. These volunteers are making a real contribution to human knowledge."

For more information on photo tagging, go to www.tagasauris.com