I've been really keen on special effects since my childhood, brought up on a fantasy intensive diet of Gerry Anderson productions and science fiction movies.

My son, who is a teenager, recently watched Ghostbusters for the first time, and commented on how some of the special effects aren't exactly perfect. He asked me to explain how some things were done, and being a dutiful geek father I explained some of the subtle and not unsubstantial technical challenges that were met by this picture in the pre-digital era.

For example, he was curious about Mr Stay Puft, and amazed to discover he's a man-in-a-suit. But the apparent 100ft scale Stay Puft  is reinforced by small touches like the fire hydrant he knocks over in the street. Using water would have looked out of scale, so what is fired out of the hydrant is actually very fine sand, which looks perfect!

But this discussion set me thinking about modern effects, how good they are, but also what they might have robbed us of.

Looking just at this summer's blockbusters, the effects now on show are quite stunning. It's now possible to create starships that move gracefully through space with scale and grandeur, while amazingly complicated robots interact with human actors seamlessly. It's all technically stunning. Except deep down in my geek heart, do I believe any of it?

The problem as I see it, and it's maybe a personal issue I've got here, is that at whatever subconscious level, I believe very little of what I see now in movies, which makes for less emotional involvement, I'd suggest.

The trigger for this thinking has been me presenting what I consider important films to my own children, and then having to explain to them that in many instances what they're seeing onscreen is real to a degree, however implausible.

I've considered a few films that demonstrate what I'm talking about, the first of which is the wellspring of movie car chases Bullitt (1968).

This won a film editing Oscar, but what people remember is the fantastic car chase where Steve McQueen pursues the bad guy's Dodge Charger in his stunning 390 CID V8 Ford Mustang. How many times have we seen this since? A million, so why is this special other than it was the grandparent of them all?

It's special for me because when I watch it I know that, although some of the vehicles on the road are ‘controlled', not all of them are. What we're seeing here is in essence a real chase, performed on the streets of San Francisco early in the morning. The cars are being driven to their very limits by two people who know exactly what they're doing, and one of those happens to be the film's star, Steve McQueen.

Imagine today the script for Bullitt was green lit, and the studio went to their backers and said ‘we're having a car chase where we'll be going over 100 miles an hour, and we'd like the star of the movie to do his own driving'! The idea would be vetoed before the sound of that sentence had travelled across the room, because no insurance company would underwrite the risk. These days you just accept that the star isn't driving the car, flying the plane, jumping out of the helicopter, whatever the film suggests. They're living in the very life threatening world of being in the proximity of a green-screen, and that's how close to reality the studio wants.

But even if I know that the star isn't the one in jeopardy, it gave films a certain edge when you knew that someone was putting his neck on the line to get that shot. The two that always stick in my mind are the train explosion in Butch Cassidy And The Sundance Kid, and the high fall that is at the beginning of Hooper.

In Butch Cassidy you know that it isn't Robert Redford and Paul Newman who take the full force of that wooden train carriage being blown to smithereens, but you know that some poor schmuck was picking splinters out his front for the next two weeks or more. And in Hooper there wasn't any belief that it was Burt Reynolds on the skid of that chopper in the opening sequence, but a real person (A.J. Bakunas to be accurate) falls 232 feet into an airbag, a stunt record for height that's never been broken.

Even when you accept that these aren't the actors who are portrayed doing these things, there is an inherent reality that they're real and dangerous.

The advantage of our entirely synthetic modern effects is that they're inevitably cheaper, insurance friendly, and, they'd argue, just as convincing. But as great as the Transformers look, I know they're not ‘real', and that no one was in danger of being trod on or crushed by them.

But I recently watched The Battle Of Britain, and that threw another spanner in the works of film reality. There are some quite duff model and optical effects in that production, but there are also some incredible flying sequences filmed using mostly period aircraft. For hardcore geeks, the Messerschmitt's are really Spanish Hispano HA-1112 Buchons, though they are based on the BF 109 and so look pretty good.

The point I'm wandering towards here is that these Messerschmitts and the 12 Spitfires, 3 Hurricanes and 32 Heinkels were available in 1968 when they made this movie, but now they're not.

Therefore, any WWI or WWII movie that involves aerial battles is going to be CGI now, as is any tank battle with German Tiger and Panther tanks. They don't exist. The Tigers in Saving Private Ryan were mocked up from Russian T-34s, because the real items are closely guarded exhibits in museums and there is only one in the world that runs, currently.

Maybe for the average cinema goer these things don't matter, but to me, whenever I see any sort of historical piece of this type, I know immediately that it's not real whatever effort they've made with the CGI to convince us otherwise.

I'm not suggesting for one minute that they stop making historical drama, but these things to a geek are the acting equivalent of breaking the fourth wall, i.e. the actors turning into camera and winking at the audience. Except in this case it's the special effects people knowing that they're presenting something impossible, and that a good portion of the audience will know it's fake precisely because of that.

Perhaps in this particular instance the justification for CGI is greater than most of the uses we see today. We see it now lavished to avoid locations in hot places or having dust blown by a helicopter into the eyes of the leading lady. The bigger problem comes to my mind when it's used to fix everything, and the virtues of real locations, stunts and lighting aren't even a true consideration.

Later this year, Avatar will open which, according to some sources, has the most amazing special effects of any motion picture so far. My concern is that I'll love the story, characters, the visual design and the execution. But part of me just won't believe any of it, however well it's done.

Has the knowledge of how effects technology works and watching the evolution of this art form robbed me now of the ability to just accept what they're doing without thinking about the deeper reality or inherent lack of it? In some respects it's like the difference between conventional actors and those who follow the ‘method' techniques. Some actors learn the lines and just turn up to deliver them, while others insist on inhabiting the character through experiencing physically or emotionally what they've gone through.

I've always been a fan of conventional actors, because they don't need to be stuck in a freezer for an hour beforehand to sell the notion that they're cold on screen. Yet for non-performing components in movies, I don't see that any CGI could ever be a substitute for a real Spitfire, train wreck, explosion or 20,000 costumed extras advancing at Waterloo.

But it seems that we've lost those things for exactly that, and I don't think we got a good deal.