Those of you reading Richard Franklin's impassioned review of the 3D Imax version of The Polar Express may be, if you have never experienced the process, wondering just what is so different or so special about Imax. Or even what the hell it actually is, given that there are current only five such theatres in the UK, three of which are located in central London. As someone who has been impressed by the process but a lot less so by the products created for it, allow me to quickly explain.

Imax is not just a different type of cinema, it is a different film process, and a rather special one. First up, you cannot show an Imax film in a regular cinema – Imax screens are HUGE, typically between 20 and 30 metres wide, and if sit anywhere near the front, as I do, then your entire field of vision is filled with the images on screen. Thus you are not so much watching an image as experiencing it. Secondly, Imax films are shot not on 35mm, as are the majority of films viewed in regular cinema screens, but on 65mm negative stock and projected on 70mm. This is not the 70mm of Laurence of Arabia and 2001 – Imax uses 15 perforations per frame as opposed to the standard 5, giving a frame size that is ten times that of the standard 35mm frame. The corresponding increase in picture detail is genuinely eye-popping. Thirdly, Imax theatres are designed so that the picture and digital sound is of equal quality wherever you view it from – there are no bad seats or spots where the experience is lessened (though I still recommend the front three rows). Imax cinema seats are steeply stacked – the back row is way higher than the front.

3D Imax is a process involving a specially designed camera, a 'rolling loop' projector and large 3D polarised glasses. The process is genuinely extraordinary. Having seen and thoroughly enjoyed Jack Arnold's The Creature From the Black Lagoon and It Came From Outer Space in 3D (and, I might add, at the Piccadilly cinema which dissolves in violent mayhem at the climax of An American Werewolf in London), I was still not ready for this. Whereas previous 3D processes had produced a convincing 3D effect, creating the illusion that objects extended from the screen (the opening explosion of a descending meteorite in It Came From Outer Space sent a young John Carpenter screaming from the cinema, but also began his love affair with the movies), compared to the Imax process this was primitive stuff indeed.

3D Imax has actual physical space – it can place an object at a precise distance from your eyes. Thus, if the film-maker wishes to put something a foot from the end of your nose they can do it. My first experience of this was not so much disconcerting as startling – I was able to actually put my hand through an image that my eyes and my brain were telling me was sitting just in front of my face. The sense of depth this creates is genuinely remarkable, and combine it with an image that fills your entire field of vision and this ceases to be a film and becomes an experience, just one motorised chair away from a top level theme park ride.

Which is – or at least was – precisely the problem. Early 3D Imax films were visually striking, not necessarily for their lyrical photography, but because the 3D Imax process is so good that EVERYTHING looks remarkable in it. The traditional movie skills of storytelling and character were simply deemed unnecessary, as all you had to do was find something that looked spectacular on screen, tag on an introductory sequence and off you go. Thus you could go to an Imax screening that, like the early Cinerama demos, consisted almost entirely of a camera bolted to the front of a racing car as it whizzed around the track. And visually it was something else – you leaned as the car hit a curve, you lurched back in your seat as you approached other vehicles. Which is all well and fine, except I had that exact same reaction to the subjective shot in Star Wars when the X-Wing flies into the Death Star trench, and yet when I went to see that I also got Star Wars, which had plot and character and humour and storytelling. And it was a lot cheaper to get in.

Consider my first 3D Imax experience, a film tantalisingly titled The Hidden Dimension. It begins with a cheery, smiling, all-American family arriving at the empty lakeside house of an elderly relative. The father has work to do, so the young daughter goes exploring and finds three keys, each of which relate to science and eventually a device that opens up to her the microscopic world of insects. The next twenty-five minutes consist of moving three-dimensional camera shots over gigantic microscopic enlargements of insect life. Technically fascinating, of course, but once you've seen three insect heads blown up to twenty metres wide, you've seen enough to last you a lifetime. Shorn of the 3D, The Hidden Dimension would be a pretty miserable excuse for a film – plotless, characterless, and one whose only real entertainment value, at least on my viewing, came from hearing its American cast dubbed into throaty Japanese (well, I was in Japan). In the spirit of Marshall McLuhan's 'The medium is the message', we were being asked to marvel not at the film but at the technology that produced it. Like a firework display, it was visually impressive, but lacking any real substance.

Whether the 3D Imax version of The Polar Express falls into that category is a matter of opinion – it is certainly much more than a series of images slapped together to promote the 3D process, a genuine movie in the sense that it creates characters and tells a story. That it apparently uses the 3D Imax process to eye-catching effect doubtless makes that version the one to experience, but given the fact that just about everything looks amazing in this process, surely the best place to judge the film in the more traditional merits of how well it tells its story and defines its characters is when it is divorced from the razzle-dazzle of this technical presentation. In his review, Richard makes the point that the film should not have been released in normal theatres, and he may well be right, but the fact is that it was, and that is precisely where the vast majority of its potential audience are going to see it. And having handed over their hard-earned cash, they have the right to expect a film that works in the very environment that the expensive "in cinemas now" advertising campaign has encouraged them to attend. That they have not seen the 3D Imax version is not fault of their own given the tiny number of Imax theatres in the UK and the cost of getting to and into them. I live in an economically depressed area, and an outing to see The Polar Express at the cinema is expensive enough for most low income families, but for the eighty-mile-plus family trip into the centre of London, complete with ticket costs and meals, you could take the kids to Tenerife for a few days. It's just not a viable option. This is not because they "can't be bothered" to see the Imax version, it's a simple matter of access and economics.

As a primarily DVD-driven site we are repeatedly faced with the difference between how a film played in the cinema and how it looks on the smaller screen. And there is a difference. Take Michael Haneke's Funny Games as an extreme example. A film that challenges its audience to question their own attitude to screen violence, it is an emotionally devastating experience in the cinema, and though much of its power remains on the small screen, it is a much 'safer' viewing experience. When it gets too much we can talk ourselves through it, look over at the clock on the wall, nip out to make a tea or even fast-forward through the nastier bits. In the cinema, however, you are trapped by the experience, and if it proves too much you have only one recourse, and that is to leave, precisely the reaction Haneke was looking to provoke. And yet even given all that, the film's success as tool of audience manipulation and its painful identification with character can still be appreciated on the small screen, something that simply cannot be said for The Hidden Dimension or many of the early Imax films (some of which are ludicrously available on DVD if you want absolute proof).

Which brings me back to It Came From Outer Space, which looked great in 3D, but also works just as well in a regular cinema and on TV and on DVD. You know why? Because it has interesting characters, it has atmosphere, it has plot, its has a wealth of subtext about identity, McCarthyism, Communism, however you want to read it, and it's a thrilling, inventive, low-budget science fiction treat. It works in whatever medium you want to view it because it is, quite simply, a MOVIE.