Monochrome mummers: A retrospective of the theatrical photographs of Nobby Clark has just opened in London. Marina Benjamin on the artist and his methods

Nobby Clark has been snapping stars of the stage for 25 years. With a knack for putting his subjects at ease, an eye for the spontaneous moment, and a reputation for getting pictures the paparazzi would kill for, Clark has a lot to be proud of. But he wears his success lightly: 'Photography should be fun - but if you're flippant, it's assumed that you don't take your work seriously.'

Taking Clark very seriously, London's BAC is hosting a major retrospective of his theatre work. Vanessa Redgrave, Placido Domingo, Al Pacino, Helen Mirren, Bruce Springsteen and Max Wall are just some of the characters featured in the exhibition. But Clark's oeuvre is far more than a celebration of luvvies down the ages. For he has found the entrance to the secret garden, sneaking behind the scenes to capture off-guard dressing- room antics, angst-ridden rehearsals, and priceless moments of merriment and mirth.

There are wonderful images of Barry Humphries commencing his transformation into Dame Edna, Robert Lindsay as Cyrano acclimatising to the hilarity of his nose, Jessye Norman centre-stage in an empty, chair-strewn auditorium, David Mamet intently poring over a script, and Steven Berkoff tearing his hair out - apparently in response to Antony Sher.

'I love taking pictures at rehearsals,' says Clark, 'there's a magic there that sometimes gets lost on stage.' Clark's modus operandi centres on becoming one of the team. He's present from the read-through to the dress rehearsal, winning the trust of cast and crew along the way. When you meet Clark, it's easy to see how he's managed to achieve unparalleled off-stage access: he's easy-going, affable and completely devoid of pretensions. 'I came into this world out of nowhere,' he announces. To be precise, his unlikely gateway to photographic circles was his father's pub in Pimlico. That was where Michael Croft invited him to photograph the National Youth Theatre's 1969 production of Zigger Zagger, and where budding photographer, Patrick Cocklin, offered to share his Smithfield studio with him.

But he got his real break in the Seventies when he was hired by the Observer. Clark has a distinctive and uncompromising style: he works with available light, uses fast film and fast lenses, and has a penchant for over-exposure and over-development, all of which make the blacks blacker and the whites whiter. 'I'm known as dark and moody,' he laughs, 'and I'm often told my negatives are unprintable.' But Clark can print them: 'I can photograph a production that's pitch black and still get something out it.'

Theatre photography has never enjoyed the status of fine art. It is assumed (navely) that the innovation is on the stage rather than behind the camera. Yet Nobby Clark's mastery of dramatic chiaroscuro - his ability to make as much of darkness as of light - ought to rank him among the best practitioners of black-and-white photography.

Nobby Clark's Theatre, BAC Gallery, 176 Lavender Hill, London SW11 to 17 Jul, free

(Photographs omitted)