I once spent an evening looking at a single photograph of a lily. Robert Mapplethorpe took it in the 1980s. On the surface, it is only a flower, lit and framed with total control. But something else sat behind that image, and I could not work out what it was until I read the writer and teacher Terry Barrett.

Barrett built a formula for how photographs create meaning. It is simple to write down and hard to unlearn once you know it: subject, plus medium, plus form, plus context, equals content.

Subject and subject matter are not the same thing

Barrett makes one distinction that changed how I look at photographs. The subject matter is what the photograph shows. The subject is what the photograph is actually about.

Take that Mapplethorpe lily. The subject matter is a flower, photographed close up and in exquisite detail. The subject is bigger: beauty, sexuality, mortality, the line between something grown and something built. The flower is not the point. It is the vessel for the point.

Once you separate the two, you stop cataloguing what you see and start asking what the photographer wants to tell you through it.

Medium: how you meet the photograph matters

Barrett wrote at a time when photography mostly meant a physical print. Now most of us meet a photograph on a screen, and that changes the medium in ways he never had to account for.

A photograph on a phone, seen for two seconds on a train, is not the same photograph hung four feet wide in a gallery. The size, the light, the place you are standing in when you see it: all of that becomes part of how you read the photographer's intent. The medium is not paper versus pixels alone. It is the whole experience of viewing.

Form: the choices behind the image

Form is everything the photographer decided on: composition, light, colour, texture, depth of field. Barrett asks us to notice how these choices support what the photograph is trying to say.

Here is a way to test it. Imagine shooting a fever dream as a photo story. You would probably want broken compositions, colours that jar, angles that should not work, and a sequence that refuses to settle. The form would carry the disorientation itself. Every choice would do some of the work that the subject is doing.

Context: the life behind the lens

Context is the photographer's biography, their moment, their influences. Mapplethorpe was a gay artist working through the AIDS crisis, trained in classical composition. Once you know that, even his flower studies carry more weight.

Context also reaches outward, into the history of photography itself. What is this image answering back to? What is it pushing against? What does it say about the anxieties or the celebrations of its time?

Content: where it all lands

Content is not a list of what a photograph shows. It is what happens when subject, medium, form and context meet each other on the same page.

I still think about that lily. Knowing Barrett's formula did not make the photograph simpler. It made it harder to look away from, because I could finally see how many decisions were stacked inside one flower, one frame, one print.