Essay: The outer-space documentary as big as the Ritz

Every new medium goes through a stage where the point is the technology rather than any one story it might tell. The film-goers in the 1890s who sat rapt before scenes of horses trotting and people walking, and screamed when a train seemed to be coming out of the screen at them, were not being naive so much as registering the crude power of the cinematograph to overwhelm our senses, at a point in the development cycle when it had as yet no other powers. In fact, there's an advantage to witnessing a technology at its early stage. You see the nature of its particular form of representation, raw. The reckless early-adopters of TV in the 1930s may not have got many programmes that were worth watching. But they did get an insight into the nature of the new medium, with its different grammar and different possibilities. We can revisit their standpoint by checking out, not television or film in its era of sophistication, but media presently at the form- over-content stage - Imax, for example, as displayed for the last month in the BFI's new theatre on the South Bank in London.

Of course, there's no guarantee that Imax is a technology destined to thrive. So far, no studio has been willing to bear the expense of shooting an actual feature on the special 70mm film stock, and the back catalogue of movies can't be transferred to the Imax format because of a straightforward incompatibility of picture shape. The top and bottom of the images required to fill the big screen simply do not exist in La Dolce Vita, or Citizen Kane, or The Seven Samurai. Fellini, Welles and Kurosawa didn't collect the right amount of light.

Imax consequently has to get by on a diet of documentaries; documentaries - no surprise - about very large things. Just as it was never exactly a sign of an individual artistic vision at work that in cheesy 3D films people were constantly pointing, and lowering ropes down wells, and generally adverting to the presence of the third dimension, so it is not evidence of a shared sensibility that the directors of Imax flicks are all so interested in volcanoes, dinosaurs, and outer space. The South Bank Imax endlessly shows a Nasa promo called Destiny in Space, narrated by Leonard Nimoy, and Into the Deep, a tour of the wonders of the sea. Neither of them are the kind of thing that would hold your attention if chanced upon when channel-surfing at home. In fact, in terms of content, they reminded me of the information films they showed at my school on rainy Friday afternoons in the 1970s: films like Our Cheese Industry and Ghana - The Cocoa Kingdom. The Imax gamble is that these things will affect us again (or rather, affect us with different, better emotions) when magnified so they are not just larger than your telly, but larger than your house.

In place of narrative excitement, the South Bank Imax offers a sense of occasion. It's a sleek, steel-ribbed glass cylinder rising from the underpass at the centre of the roundabout at the south end of Waterloo Bridge. Formerly Cardboard City, Waterloo's shanty-town of homeless people, this O of ground has been captured by the forces of techno-optimism. The building smells of new carpet on a spot which once smelled of piss and policy failure. It looks like the kind of really expensive audio-speaker that is designed by a Finnish ergonomicist who favours rimless glasses and Nehru jackets. You process upstairs. The auditorium is a transverse slice of the whole cylinder, with the seats steeply raked so there are no heads between you and the 20-metre-by-26-metre screen wrapped around the curve of the opposite wall. A BFI person stands at a microphone, a small light shining on his face, and delivers a health warning which must surely be intended in part as a come-on. Carnival barkers down the ages have promised that their ride will give you a heart attack or turn your hair white. "Some people can find the Imax experience overpowering," he says. "If an image is too much for you, shut your eyes for a moment until the image changes."

And then you're off on a ride which is a dizzy mixture of true grandeur and sudden bathos. Much of Destiny in Space was filmed in the early 1990s by astronauts aboard the space shuttle: the view from orbit has never looked so all-embracing, so vertiginously, truthfully world-sized. Across the foreground moves the cream flank of the shuttle, revealed by the fine analog grain of the film stock (no pixels here) to be an airframe bandaged over with many separate composite panels, without doubt a made thing, the fruit of ingenuity. Far, far above, so that you have to turn your head to see it, the blue-white limb of the Earth cuts across, its line a perfect arc which equally clearly belongs to a different order of creation. The cloud systems crossing the oceans down below have a feathery clarity. A view like this reminds you that "breathtaking" is a metaphor about the body. When an image is this large in scope as well as in area on the screen, it squeezes a silent gasp out of you, like the collective "Aah" forced from our throats at a firework display. On the other hand, when the Imax is displaying things that aren't intrinsically giant, the size of the image registers as a grotesque inflation, and anticlimax swiftly follows. I saw a simulation of the surface of Venus fly by, the red lines of mountains and volcanoes swollen with the gold light of upwelling lava; I also saw, when they interviewed the originator of the Hubble Space Telescope, a stringy, amiable 80-year-old expanded to become the world's largest pensioner.

The absurdity that's generated when the Imax is working with images on the human scale has a straightforward perceptual cause, I think. The crucial fact is that the screen is larger than the central portion of your eye's visual field - the section of your vision within which you can grasp things at a glance. More is always going on than you can immediately perceive the unity of. You look around the image rather than simply gazing at it. This is a very different regime from the one that usually obtains in film; and I imagine there's another obstacle to Imax being used for a feature. It thwarts intimacy with the images on screen.

Instead, Imax makes you react as to a panorama. It compels you to recognise, not just the size of the thing imaged, but your own scale in relation to it; metaphysically speaking, it is always placing you on the lip of the Grand Canyon. All of the South Bank Imax's promotional literature emphasises the immersiveness of the experience. "You'll float up into space, feeling the wonder of weightlessness ... " But I'm not sure that's right. As your point of view floats in low earth orbit, you don't feel horror vacui, the fear of the emptiness around you, or even the milder fear that grips you when you're swimming in the sea, and you come to a sensuous appreciation of the depth of dark water beneath you. You know that volcanoes of Venus will not burn you, the deserts of Mars will not freeze you. The Imax gives you the very particular pleasure of having danger unfolded to you so that you know your own smallness in the face of it, yet at a distance that completely assures your safety. There's a name for this. As so often, the best description of a technology precedes the technology itself by a couple of centuries. "It fills the mind with grand ideas, and turns the soul in upon herself," wrote Edmund Burke in 1757. Imax is what he said thunderstorms and ruined castles were: sublime.

Francis Spufford's documentary, 'Flying Spitfires to Other Planets', about the forgotten British space programme, will be broadcast on Radio 4 in July.

BFI London Imax, SE1 (0171 902 1234)