There's a clip in tonight's BBC documentary 100 Years of Wildlife Films that's like nothing you've seen before. That may sound like an exaggeration in an age when cameras can take pictures in ocean trenches and termite mounds. But this is something you just don't see on television.

It's from the 1928 film Simba, made by Martin and Osa Johnson - he a former cook, she a former cabaret singer - who were at the time America's most popular wildlife film-makers. They are seen perched on a Jeep, dressed in safari suits, Osa with cinched-in waist, thrust-out chest and fetching tin hat. Not far away there is a lion. They get unobstructed footage of the lion killing a zebra, remarkable in itself - but then the camera turns on Osa. She takes her rifle, raises it carefully to her shoulder - and shoots the lion stone dead. Osa smiles and preens before jumping down from the Jeep and posing with the dead animal.

"It's incredibly shocking," says Tim Martin, the series editor of Natural World, who made the 100 Years special. "But you have to look at it in context. Some of our greatest conservationists were hunters. It shows how our relationship with wildlife has changed."

Today, natural history films are arguably doing more for the cause of conservation than any campaign, but in one sense they are still closely related to the Johnsons' work - in the sense of being among the most enthralling things on television. "I watch them all the time," says Sir David Attenborough during the centenary programme. "It seems to me they are the most exciting and remarkable viewing you can have. They are beautiful, dramatic and, above all, true - they're not selling anything. If you're tired of natural history films, you're tired of life."

Tonight's programme celebrates this genre and the individuals who have brought to film everything from the springtail (a half-millimetre insect) to the mighty blue whale. Wildlife film-making is in the middle of a golden age, especially in Britain. But there is also constant pressure on film-makers to keep offering new perspectives on the natural world; to strike a balance between truth and audience squeamishness, and to do so with ever-decreasing budgets.

It was 20 years before Attenborough's birth, and only six after Queen Victoria's death, that wildlife film-making was born, with the help of a can of pebbles. Oliver Pike, a keen British nature photographer, headed for the remote Atlantic archipelago of St Kilda to film the gannets and puffins - and immediately became the first of a long line of cameramen to be let down by their equipment. When he turned the handle on his heavy mahogany camera, it made a grinding noise so loud that any bird within half a mile took flight.

So Pike launched an honourable tradition: he improvised. He put some pebbles in a can and spent days sitting a few feet away from the birds, shaking it. At first they took to the air, but after a while they learned to ignore him. After that, when he started to roll the noisy camera, they didn't bat a feather. The wildlife film had arrived.

Things advanced rapidly. In 1913, the American John Ernest Williamson took the first underwater pictures, from inside a diving bell. In 1934, slow-motion photography was pioneered in a British film, The Private Life of Gannets. And just as the technology was evolving, so were the presenters. While the Johnsons were making themselves the stars of their own films in America, in Britain the dominant figure was the wildlife enthusiast Cherry Kearton. He had a tendency to anthropomorphise the animals, but kept the focus on them. Attenborough recalls him as a boyhood inspiration and "one of the great pioneers".

Another was Sir Peter Scott, the son of Captain Scott of the Antarctic. While awaiting death in his tent on the ice, Captain Scott wrote to his wife that she should instil in their son a love of wildlife. Peter became the first person to present a live wildlife programme, and founded the conservation organisation WWF.

Indeed, he epitomises the changing ideas of the time. He had been a hunter of wildfowl, but gave it up after watching a male goose stand for hours next to a female that was dying of a gunshot wound. Growing respect for the natural world was pushing out old established attitudes.

It is David Attenborough, however, who has defined the genre. He started out in Zoo Quest - about catching animals for zoos - in the 1950s, but went on to present the likes of Life on Earth, Blue Planet and Planet Earth, films that set a new standard.

Gordon Buchanan is a Glasgow-based cameraman who has worked all over the world filming tigers, lions, pandas - and, in June's Springwatch, foxes in Glasgow. He remembers lapping up Attenborough's BBC series, and ITV's Survival, as a child. For him, the fact that wildlife film-makers have continually reinvented the genre "to constantly provide films and programmes that people are still passionate about" is one of their greatest achievements.

The Natural World strand on BBC2, which runs to 17 programmes a year and has been going for 40 years, epitomises this spirit of innovation. Its most recent series included a film about the co-existence of humans, bears and moose in Alaska, and the heart-rending story of Toki the orphan cheetah, adopted as a cub with his brother Sambu (now dead) by Simon King, the presenter better known for Springwatch. The film showed how King is struggling to protect Toki from the many hazards he faces living wild on the Kenyan plains. "We might have an animal drama one week, a sciencey film the next, then a human-based one. They will surprise you and you know they'll be good," says series editor Martin. "We do take risks."

Gordon Buchanan says that a good editor is one of the most important contributors: they can take film from a shoot that didn't go well and create a compelling narrative. Soundtrack and narration are crucial. "The music can convey so much that words can't," he says.

It's tempting, of course, to overlay a great film with an urgent narrative about habitat conservation. Alastair MacEwen, a veteran film-maker who has worked on such programmes as Survival and Life on Earth, says conservation is uppermost in most film-makers' minds. "But the thing you have to be aware of is that, as soon as you start talking in earnest about protecting things, people become less interested. So the attitude has always been that if you try to tell an audience about a beautiful place with animals you'd sorely miss, that in itself is a conservation message."

The imperative for editors is audience appeal. "There has to be some sort of a spin - a well-known presenter, for example," says MacEwen. "So the decisions made today are a bit different from what they were - if it's small and it's brown, then no."

If it's large, brown and covered in blood, then yes, but with caveats. How much violence to show is a classic dilemma. As Attenborough says: "It's a very narrow line. You can't in my view eliminate it entirely - that is to sentimentalise and distort reality - but, equally, some of it is very hard to take."

MacEwen agrees. "You see things you would never turn the camera on to because it would never be transmitted. It turns your stomach." The bitter truth, he says, is that animals are often still alive when a predator starts eating them - a lion, for instance, can subdue its prey just by putting its paws on it. "You're not going to watch a buffalo calf being eaten alive."

Buchanan is a little more ambivalent. "It's a tricky one, because I personally would like to show things warts and all." He compares it to childbirth: "We all know what's involved, but that doesn't mean we want to sit down and watch it".

Film-makers have to be prepared to get close to predators - sometimes alarmingly close, it seems to the viewer. But Buchanan, who has tracked tigers on foot, believes there is much misunderstanding about the risk animals pose. "It sounds dangerous, but if you know the animal and what it's likely to do, it's much safer." He says he is more afraid getting into a car with a stranger behind the wheel.

There is no shortage of people keen to make wildlife films. "Bristol HQ of the BBC's Natural History Unit is the world capital of wildlife film-making," says Tim Martin. There are probably more UK-based wildlife film-makers working full- time than anywhere else in the world - between 50 and 60 at any one time, according to the London-based International Association of Wildlife Film-Makers (IAWF).

Buchanan says he was working in a hotel on Mull when he was taken on as a cameraman's assistant. After 18 months, he tried going it alone and spent two years going further and further into debt, before a lucky break led to a steady stream of commissions. Nevertheless, says the IAWF, it is becoming harder to get work in the industry: budget cuts mean few film-makers are able to afford assistants any more, even though demand for the programmes has grown. Put simply, they are expected to do more for less.

That means fewer days to get shots of elusive animals. But that just makes the triumphs even sweeter. "The sense of success is overwhelming," says Buchanan. Once filmed, never forgotten - that's the essence of great wildlife film-making."

  • 100 Years of Wildlife Films is on BBC4 tonight at 7.10pm.

Five scenes we'll not forget

  • Sir David Attenborough and the gorillas (Life on Earth, BBC, 1979). We held our breath with him as he edged close to the gorillas in Dian Fossey's sanctuary, Rwanda. Attenborough had been intending just to narrate a piece nearby - but, as he crawled forward, he found himself next to a female. Discarding his script, he whispered: "There is more meaning and understanding in exchanging a glance with a gorilla than any other animal I know."
  • Chimpanzees hunting (The Trials of Life, BBC, 1990). Uncompromising film of a troop of chimpanzees hunting a colobus monkey, some herding him, some serving as lookout and others waiting patiently to ambush him. Chilling but fascinating.
  • Killer whale toying with seal pups (Blue Planet, 2000). Similarly unsettling footage in which a whale comes out of the surf to grab seal pups, which it proceeds to toss around like a cat playing with its prey.
  • The Okavango Delta (Planet Earth, 2006). The Kalahari desert is one of the most unforgiving environments on earth - except during the seasonal bloom, when the basin becomes a lush paradise. Baboons, elephants, hyenas and jackals trek for miles to reach it, resulting in the closest thing to sheer joy you're ever likely to see in the animal kingdom.
  • Death of a polar bear (Planet Earth, 2006). The film tracked an adult male who had woken from hibernation to find the sea ice had receded so far he could not find any prey. He was forced to swim for days until he found a walrus colony. Exhausted and desperate, he tried unsuccessfully to kill one. Then, starving and injured, he lay down feet away to die. A heart-breaking indictment of global warming.