With his most recent project, Portuguese-born visual artist Edgar Martins set himself a challenge: to capture absence – or, to put it in slightly less abstract terms, to photograph the experience and reality of incarceration without taking any pictures within prison walls.

Martins refers to each work in this exhibition not as a photograph but as a “lipograph” – “a visual representational device that omits its referential subject”. Taken together, he suggests, the images turn traditional documentary photography on its head, revealing “the hidden narratives rather than the glaring truths”, and functioning as an act of resistance in the face of dominant perceptions of imprisonment and inmates.

The primary purpose of prison is (or should be) not punishment but rehabilitation – or, as the slogan on a trucker’s cap in one photo puts it, to “Make Men Great Again”. But paradoxically, for many inmates, life after release is not liberating. In the pictures taken immediately outside prisons, Martins’ subjects look dwarfed by the scale of the walls, lost, daunted by the prospect of going it alone – pining, perhaps, for the companionship and camaraderie of the carceral community from which they are now excluded.

Even more striking is the image of a man appearing to pull a length of metal chain out of his mouth – on one reading, a powerful visual metaphor for the painful process of freeing oneself from the prison system. But it’s a still photograph rather than a video, so perhaps the reverse is true – is he actually swallowing the chain, and Martins is instead depicting the process by which incarceration is internalised to become a state of body and mind?

Of course, imprisonment affects not only those deprived of their liberty but also those deprived of their loved ones. Arguably the most poignant picture on display is of a single white sandal, with the label “DADDY’S GIRL” affixed. Echoing the tragic six-word story “Baby Shoes” most commonly attributed to Ernest Hemingway (“For sale: baby shoes. Never worn.”), it underlines how keenly absence(s) can be felt.

Ffotogallery, Cardiff, until Sat 9 Apr. Info: here

words BEN WOOLHEAD