It's a surprising fact but the Antarctic is actually easier to visit than the Arctic. Warmed by a hot direct sun, the Antarctic is positively toasty compared to the damp and dingy North Pole. Even in summer, when it's bathed in sunlight 24/7, the Arctic doesn't have enough light to power a solar panel. In spring, the temperature hovers at around -40 degrees Celsius.

That's something Martin Hartley knows all about. He just got back from a 60-day trek across the Arctic, helping two members of the Catlin Arctic Survey document the effect of increased carbon dioxide absorption in the Arctic Ocean. "One of my tasks was photographing the state of the ocean itself, because some scientists say it will disappear in our lifetime," he says. "I felt a huge responsibility about that."

Hartley also needed to document the team's work and to take press and PR pictures of the trip, and he took two cameras to get all the work done - a Nikon D3x and a Leica MP. The Nikon was mainly for the press shots as they could be sent back to the UK each day, but the Leica was a fail safe - at low temperatures and high humidity, a manual camera is often more reliable than a digital SLR.

"The cameras usually have to be left outside the tent, which means they have to be able to withstand temperatures down to -50˚C," says Hartley. "When things get that cold they contract. Electronic focus doesn't work because the lenses get too tight, leaf shutters seize up and the mirror can also freeze. When it [the frozen mirror] goes up it stays up, you have to wait for it to warm up to drop down.

"But worse than the cold is the humidity. If a DSLR comes into the tent at night it has to warm up very slowly in a bag of silica gel with a towel wrapped around it, so that any condensation gets absorbed rather than going into the electrics. If moisture gets into a lens - which can be unavoidable - you have to know how to get the water out otherwise it will turn to ice and you'll get little ice crystals forming. It's a juggling act to keep things working for 60 days."

Battery power is also an issue with digital cameras, as they drain 200% slower than usual in the cold. Hartley took 16 batteries with him, and had to carry them in plastic bags inside his clothes to keep them warm and dry. He then had to take the necessary batteries out each time he wanted to take a shot, replacing them inside his clothes once he'd got it. If they fell onto the ground they were finished - the Arctic is salty as well as icy.

The Leica MP, by contrast, was tested down to 35˚C by the manufacturer with very few problems and, with no batteries and no electrics to seize up, could be carried outside Hartley's clothing in a thin plastic bag. He still had to be careful with his film, however, as it becomes brittle and prone to snapping in sub zero conditions. He warmed the camera up each night over a stove to put the film in and take it out, gingerly winding it on by day. But Hartley was also keen to use film for its archival properties - he needs his Arctic Ocean shots to be archived for future generations, he points out, and with film he knows for sure it's safe.

Even still, he says DSLRs serve a vital function on modern expeditions. "I had a special computer designed, which used two iridium modems to send images back every night at 1MB/hour," he says. "I'd send about five or six images per night, just stuffing the Flash cards in and leaving it running while I was sleeping. There was no monitor, nothing, but the pictures appeared in the national press while I was away. The next best thing to that was using a satellite iridium telephone-based PDA [a palmtop computer], which takes about 20 minutes to send 200KB. It's great for images destined for websites."

All in all Hartley's kit weighed 38kg, and he also carried his tent, food and equipment. It made for a punishing 60 days, but for Hartley photography goes hand in hand with this kind of adventure. He specialises in documenting inaccessible places and has been on 19 polar assignments so far. "For me, exploring and photography came at the same time when I was five years old," he says. "I was given my first camera in an adventure kit."