In architectural jargon, verticality is king.

In architectural jargon, verticality is king. From the Great Pyramid of Giza to the once-towering mediaeval spire of Lincoln Cathedral and on to London??s Shard, we have always built taller for impact, prestige and ground-space economy.

It was prestige and land pressure that guided the conception some 50-odd years ago of what was once Africa??s tallest building, Ponte City in Johannesburg, now the subject of an exhibition by South African photographer Mikhael Subotzky and British artist Patrick Waterhouse.

The cylindrical, 54-storey tower of luxury residential blocks and leisure spaces ?? completed in 1976 in an exclusively white neighbourhood to the backdrop of the bloodily subdued Soweto uprisings ?? has become an icon of South African architecture, built to take maximum advantage of fabulous views of the surrounding city. Yet in its own curious way, with its inner windows facing onto its hollow core, it also reflected the wilful blindness of the apartheid society that spawned it.

??Even by the time it was completed, what Ponte promised in terms of utopian lifestyle was being tested,?? says Scottish National Portrait Gallery Curator of International Photography, Anne Lyden, who is supervising the installation of images Subotzky and Waterhouse are nailing ?? quite literally ?? on the walls, right up to the gallery ceiling.

??For many people it was unnerving to be in this urban tower as the county went through these massive changes. By the mid 1990s and the end of apartheid, the demographic of the residence had changed. There were more people moving into the city from the surrounding townships and a huge influx of immigrants from other African countries.??

By the 2000s, Ponte City had become home to drug dealers and brothels, its hollow core a well-known suicide spot. Bought in 2007 by a development company promising a ??New Ponte??, the developers moved in, evicting many of the residents and clearing the flats, throwing all the detritus and rubble of renovation into the central core of the building. Popular accounts say that the rubble lay five storeys deep.

It was at this point that Subotzky and Waterhouse, who had met while on a residency in Italy, began to document the building. ??There was a lot of renewed hope in Ponte at that time,?? explains Lyden, ??but when the global financial crisis hit, backing for the major renovation disappeared. The situation went from one of aspiration to one of apocalyptic degradation.??

Subotzky and Waterhouse began photographing and collecting. ??They would photograph then knock on all the doors, and if anyone answered and let them in, they would take photographs out of the windows,?? says Lyden. The residents themselves, it turned out, were often not admiring the striking views, but watching TV, which the duo duly documented on film. In the evicted and abandoned flats, the artists collected items left in the rubble, from promotional flyers advertising divorce services to personal diaries.

The results are striking, from images of decay and depopulation to those of vitality and normal family life. ??For the artists, it??s about revealing the different stories and layers in a very visceral way. The sheer volume of photographs, layered on top of one another, addresses the number of people living in the tower and the transitory nature of it,?? notes Lyden. ??People come and go. The residency list is forever changing.??

Ponte City, now, has been transformed. Bought back by its original owners, it has been regenerated and turned into a residence for the working and middle classes. ??There are a lot of myths around Ponte,?? says Lyden, of the building with a void at its heart. ??What Mikhael and Patrick have done is to venture inside and get a truer account.??

Ponte City: Mikhael Subotzky and Patrick Waterhouse, Scottish National Portrait Gallery, Edinburgh (0131 624 6200, www.nationalgalleries.org) until April 26. A free artists?? Q&A takes place today at 2pm in the Hawthorden Lecture Theatre.