I remember the photo well. I had worked for months in dodgy cafés saving up, struggled through lost tickets and torn maps, and finally there I was, camera in hand, standing in the shadows of the Colosseum. Imperious Rome, ancient and modern, rising up before me.

My trusty camera, the size of a small photocopier, was battered and bruised, but I had lugged it all the way. Miraculously, I had remembered to charge the batteries and scraped together enough lire to buy a roll of film. Twenty-four exposures. I huddled in the dark, wrapped it carefully inside the wheels of my camera, wound it to the right spot, then spent a good 30 minutes inching around the arena, like a demented gladiator waiting for the right moment.

I saw it and pounced. Sunlight was pouring through an archway, the crowds had cleared and this would be my one picture of the Colosseum before we left Rome to head for the coast. I pointed the camera, pressed the button and, as if from nowhere, a nun walked out in front of me and blocked the view. I couldn't risk another photo - imagine, only 23 left for the rest of the trip! The moment was gone and a few irreligious thoughts may well have crossed my mind.

Now when I return from a holiday, I not only have video footage and thousands of digital images, but phone images, websites I can turn to and friends who travelled with me with their equally powerful - and minuscule - cameras.

But with memory cards full of pictures shot at random, I file them... and leave them filed. It's a shocking admission from someone who treasures photography, but my brain's bad enough at jet lag and sorting through everyday experiences; why would I put the poor thing through sorting out 1,000 slightly misjudged images? They are there, safe-ish on my computer, and I will get round to them eventually.

There is always a selection that survives, but once I have posted it online, shown it to my family and zoomed in wistfully a few times, does it ever mean as much as the tactile joy of a glossy print? Something to smile at, and cry over; something to frame and crumple up in a moment of melodrama? (Guilty.)

Photos are like jigsaw pieces, moments captured that tell a story and overlay your memories. That last sunset on your first holiday. Those drunken faux-pas that will haunt you forever. Your children's early forays into holiday-making. With the clunky, old camera I had, each shot counted for so much. Each was thought through and became a matter of life or death - if I had left the lens cap on, I lost out. Sure, shots of friends and family were staged affairs, hiding more than they ever showed, but it was the memories that mattered.

What is it that I'm trying to catch now with my thousands of high-resolution snaps? When I took a video camera to Rio, I spent more time trying to focus than actually looking around me. I remember it in a haze. Perhaps on my next trip I'll leave the camera at home. Or perhaps I'll just take 24 pictures.

I remember the Colosseum better - and with more colour - from one ruined shot, than from when I returned, took hundreds, then filed them away. So to the nun who blocked my shot that day, thank you.

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