A photograph seems a simple thing: a record of a moment, a face, a place. In fact it is one of the more informative pieces of personal information a person can release into the world, and most of what it reveals is not its apparent subject.

A photograph often carries, embedded within the file itself, a quiet record of where and when it was taken. Where that record survives, an image shared casually can disclose a location and a time as precisely as a written note would. The picture was the point; the information attached to it was not, and is easily forgotten.

The visible content reveals more still. A background that seems incidental, a view, a street, an interior, a distinctive detail, can place a photograph, and therefore a person, somewhere specific. What the photographer thought of as setting is, to anyone studying the image, evidence.

Then there is the face. It is now possible, and not difficult, to match one photograph of a person against others, and so to gather images of the same individual from across many separate sources into a single set. A face has become a way of connecting things. A photograph published in one place can be linked to a photograph published in another, and the two together say more than either alone.

None of this means a person should not be photographed, or should not share images of a life worth recording. It means a photograph is not a closed object. It carries a time, a place and a face, and each of those is a thread that can be followed.

The sensible posture is simply awareness: that an image is also information, that it is often shared by others as readily as by oneself, and that an honest account of what is visible about a person includes the photographs in which they appear.