The photographer, who died in the Libyan civil war in 2011, had a gift for putting people at ease. The biggest British exhibition of his work since his death opens in Liverpool today

From surprisingly tender portraits of sleeping soldiers to officers practising golf on the front line, Tim Hetherington's photography captured moments of intimacy and absurdity in war.

His talent as a photojournalist was already evident back when I worked with him in 1998 at the Big Issue magazine in London. We collaborated on a story about homeless veterans of the first Gulf War and he soon demonstrated his rare gift for putting people who had endured great hardship and distress at ease.

Hetherington's interest in the human impact of combat was a feature of his career, which was cut short in in 2011 when he was killed in a mortar attack in Misrata, Libya, aged 40. He had established his reputation covering the human rights repercussions of civil war in West Africa and his images once provoked former Liberian president Charles Taylor to dispatch two hit squads to assassinate him.

Specialist Tad Donoho screams with pain after being administered a 'pink belly' for his birthday. Each member of the platoon strikes his stomach until it begins to bruise, hence the name 'pink belly'. Photograph: Tim Hetherington/ Tim Hetherington/Magnum Photos

But he was not a traditional war photographer or documentary film-maker. He called himself an "image-maker", reflecting how he ranged across the visual media, including video to mobile phone downloads.

His use of different media is explored in the biggest British exhibition of his work since his death, which opens in his home city of Liverpool today. The show at the Open Eye Gallery focuses on his time embedded with a US platoon in the Korengal Valley in north-eastern Afghanistan. Its title, You Never See Them Like This, is something that Hetherington said to journalist Sebastian Junger, his co-director on the Oscar nominated documentary film Restrepo, about the sight of slumbering soldiers

'Doc' Kelso sleeping at the Restrepo outpost in the Korengal valley, Kunar province, Afghanistan, July 2008. Photograph: Tim Hetherington/Magnum Photos

"They always look so tough … but when they're asleep they look like little boys. They look the way their mothers probably remember them," he said. The new exhibition includes the resulting images, published in his book Infidel, and the three-channel video installation entitled Sleeping Soldiers.

Hetherington was deeply affected by his time in Afghanistan and later said: "When I'm filming, I'm very focused … You don't really have time to start examining your emotions when you're in the middle of this kind of situation. You kind of push them to a deeper place in your mind and examine them later. But war is traumatic. I [saw] a lot of traumatic things happen in the Korengal Valley when we were there … I was with people who got killed and that was a very sad and upsetting thing."

US forces bomb insurgents in Afghanistan's Korengal valley, in 2007. The valley saw the most concentrated fighting with militants during the war. Photograph: Tim Hetherington/Magnum Photos

His 2010 short film Diary has an almost impressionistic style, collaging snatches of sound and images from various conflicts, which give a sense of the psychological and emotional impact of a decade of war reporting. A more detailed exploration of Hetherington's life and career can be found in Junger's documentary about him, Which Way is the Front Line From Here? which is released in October.

In it Hetherington explains why he put himself in harm's way, saying: "I risk my life both for personal reasons and for objective truth."

Tim Hetherington: You Never See Them Like This is at the Open Eye Gallery, Liverpool, from 6 September to 25 November.