The swimmers in Madeleine Waller's portraits stand under the leafless trees in glorious isolation. These lone warriors of the chilly morning lanes are somehow both vulnerable and intimidating. But the pictures then taken that give each swimmer's portrait a pair - of the same swimmer dressed and about to rejoin the world outside the lido - show their personalities in a very different light. By putting their clothes back on, they have rejoined society; that, after all, is what getting dressed is all about.

"I'm always struck by how swimmers have a different persona in the pool," Waller says. "I wanted to document this incredible transformation." She approached her subjects mid-swim, took the first portrait, and asked them to return to the same spot once they had dressed. The only person who turned her down was "a woman who said she hadn't shaved her legs".

The photographs were taken in winter, when the pool's populace is whittled to the hardy souls you see here, near-amphibian in their goggles and caps. Free of the jackets and boots and bags that make up our urban armour, the human form is shown in stark relief. A discreet baby bump is revealed as a wondrous stomach fit for a fertility goddess. A muscle-bound body in attention- grabbing scarlet shorts disappears, in the second portrait, beneath a bus driver's uniform.

The project is something of a love letter to London Fields lido, east London, where Waller swims regularly. Her favourite pair of photos is of John, in black trunks and gloves, for the subtleties of the light. Mid-swim, the backdrop's a fiery dazzle of sunrise; by the time he reappears in his clothes, the sky has faded to a bleak milkman's dawn.

Waller has now turned her attention to marathon runners, a project with different challenges. "I loved taking the photographs during the race," she says. "But finding the same people at the end, in the throng of thousands, turned out to be really difficult."