The photographs of Duane Michals explore the blurred border between fantasy and reality - though to Michals, `reality' itself is a fantastical patchwork of dreams and lust and fear. By Boyd Tonkin
e not forgetful to entertain strangers," counsels St Paul, "for thereby some have entertained angels unawares." But how can mere mortals, with their flawed insight and urgent longings, spot the difference between a tender angelic visitor and some predatory incubus, a guilty demon in a business suit?

This typically misty spectral drama by Duane Michals brims with the ideas that have flowed through the American photographer's work for 30 years and more - the blurring of identity, the hazy border between reality and fantasy, the sheer inscrutability of others but, at the same time, the overwhelming need to trust and touch them. And, of course, it pivots on the ancient double aspect of every human being, as ape and angel, flesh and spirit, creature of heaven and earth.

Duane Michals was 65 this year; a bureaucratic threshold of old age. But frailty and mortality - and the way that imagination or desire may overcome them - have underpinned his dramatic sequences for decades. Alongside these "photo stories", first produced in 1966, a new retrospective survey by Marco Livingstone showcases every side of a diverse career with one unifying theme. Michals has sought to turn photography into a sort of X-ray art not for the skeleton, but for the soul. That penetrating eye for the truth beyond the surface dates back to the very first pictures he took when, as a young graphic designer with Time magazine, he went to Moscow as a tourist in 1958.

The rift between documentary and fine-art photography had already opened in the 1950s. Shaped by the Surrealism of figures such as Rene Magritte - a mentor whom he visited in Brussels - Michals helped to add a third term to the debate. He transformed the photographic image into a theatre of the mind. Disputing the idea that "appearances are the only things that we consider to be real", Michals asks, "What about dreams, what about fear, what about lust, what about those intimidations which we perform on each other? These experiences, to me, constitute reality."

Michals scans human vulnerability with a searching but tender lens. Being the son of insecure Slovak immigrant parents (as was Andy Warhol) no doubt served to colour the aura of risk and hurt and need that surrounds his work. He even invented an alter ego who signs some of it: "Stefan Mihal", the married, blue-collar worker he could - or "should" - have become. As a gay man, the Pennsylvania suburbs cast him out. Yet - with his chubby, balding, clumsy body - he then entered a subculture that made a fetish out of physical perfection and so excluded him once more.

As expensively traded and commissioned photographic artists, Michals and Robert Mapplethorpe both hit the peaks of Manhattan fashion during the 1980s. In all other respects, the pair illustrate an almost textbook split between Classical and Romantic art.

Whether he pictured an erect penis or the petals of a lily, Mapplethorpe froze every image into an eerie lunar beauty. Where he is cold, Michals is warm; where he is static, Michals is mobile; where he is smooth and polished, Michals is ragged and tentative. Mapplethorpe framed his work with a Renaissance formality; Michals writes on the negatives, overlays one image on another, re-draws the line between picture and world. And because of Aids - inevitably, a recent besetting theme for Michals - Mapplethorpe's oeuvre remains forever fixed in its glacial calm. Thankfully, Michals can, and does, go on deepening his heartfelt perspectives on the inner world of men, women - and angels.

"The Essential Duane Michals", by Marco Livingstone, is published by Thames & Hudson (pounds 32)