From mourners in a funeral parlour to a delivery boy and his 'big sausage' pizza, why is it so easy to spot the opening scenes of porn films, asks Tim Dowling

These things begin innocently enough. A washing machine breaks down.

A teacher asks her pupil to remain after class. A specialist is called in to offer a second opinion. A maid comes to clean a hotel room, only to find it still occupied... Mundane moments from small, humdrum lives filled with quiet desperation.

Desperation, and horniness.

The opening scenes of pornographic films, as shown in these stills compiled by the annual magazine Useful Photography, display an almost touching dedication to basic narrative constraints. In an age when you can make a porn movie with a camera phone in a Topshop changing room, it's reassuring to find people out there who still believe in telling stories, who still think: you can't just let your characters turn up and start going at it. Where's the motivation?

The sex bits of porn films are so mechanical - and so completely interchangeable - that it's only the beginning that gives you any idea of precisely how badly made the film you are watching really is. The acting is uniformly terrible and the dialogue is worse ("Would you like a cigarette? They're quite excellent.") Taken together, these scenes resemble nothing so much as a compilation of failed auditions.

The opening moments of a porn film may provide the only opportunity for artistic invention, but they aren't usually what you'd call inventive. The situations are formulaic and rarely hampered by any flirtation with realism. Plumbers can always come out straight away, and in office scenes every day is dress-down Friday. Gay porn, judging by the evidence here, takes the least trouble with its opening formalities: two guys, a desk, a chair and a clipboard; five dudes and two sofas; a couple of young men and a chessboard. Shorn of their context, the pictures have about them a creepy, cheerless innocence, like little slices of purgatory.

More often than not, the initial setup is just a quick and convenient way to get to the line, "But maybe there's some other way I could pay you?" From that moment on, the shooting script need consist only of a series of stick-figure drawings. Even in these still photographs, you can usually guess what's coming next - in porn films no element of surprise is required, or even possible. Soon the lady carpenter will let slip an unguarded remark about needing a bigger drill. Eventually the interested parties in the car park prang will make a deal to swap bodily fluids rather than insurance details. The delivery boy from Big Sausage Pizza (Is that box the least convincing prop ever to appear in a film?) knocks on the wrong door. "I've got a large sausage here for you," he will say. Perhaps it's not the wrong address after all.

Sometimes, though, it's difficult to work out quite how they mean to get from A to B. What sort of pre-coital conversation can you have at a pumpkin stand? "Hi, Ron. That's a mighty big pumpkin you have there." "Well you know what they say, Cindy - huge pumpkin, erm... see you at work on Monday, I guess." Are scantily clad twins really going to be turned on by a goateed man playing the recorder? Far from bestowing an element of credibility on proceedings, many of these pictures make it almost impossible to believe the people in them are going to have sex.

That said, you can certainly tell these shots are from porn films, even though everybody's dressed and looking slightly bored or, in some cases, mildly horrified. Perhaps it's something about the colour - mostly primary, with a preponderance of red - or the soft furnishings. Maybe it's the pervasive air of workaday incompetence - only porn can get away with this level of shoddiness.

The classic porn film opening sequence has probably had its day. No one ever wanted to watch them, and the clips that find their way on to the internet usually have any expository scenes excised, so there's not much point in making them in the first place. But these pictures, at least, survive to document a persistent and weirdly conventional desire to persuade the viewer, as well as to remind us that inside every porn hack beats the heart of a truly terrible film-maker.

• For more information about these images, go to usefulphotography.com.