I think one reason that photographs can be such a good jumping-off point for a story is that we know they are the product of a moment. Apart from trickery in the enlarger, on the re-touching table or now on a computer, a photograph is unmediated: the camera can only speak as it finds. And it's not only photographs - images - which get my writerly self dancing and thinking, it's photography itself.
Even if the sitter has carefully chosen to represent themselves in a certain way (and that, in itself, is revealing), another moment might have made them look very different - have you ever seen a handful of snaps of yourself up on someone's Facebook page after a party, and thought you almost looked like different people, depending on angle, light and so on? A portrait is painted in time and so it's a collection, a distillation, of the artist's conception of the sitter. But a photograph is a step out of time, and therefore - paradoxically - it speaks of Time, and how it moves, as few other artefacts can.
If you've ever stood in a church where your historical character stood (as I did with Elizabeth Woodville in A Secret Alchemy) or read a notice that said "George Washington slipped here", you'll know what I mean. There's a shiver you get when you realise that but for the thinnest possible veil of time the past is here with you or you've stepped into the past. And with photographs it does come down to physics. The controlled tarnishing of silver bromide (or silver chloride or one of the other silver halides) is all that actually records the image in the camera, even with colour photographs. Photons from the sun (or now from a flash) touched that real, actual person, and were reflected away to touch the photographic emulsion on the film or glass plate, and blacken it in proportion. Of course, we're most familiar with the negative/positive process, when the print in your hands may have been made any time from the next day, to a century later. But if it's a daguerreotype, or one of its descendants such as the tin-type beloved of fairground photographers in the 19th century, then the plate is the photograph, there, in your hands.
In The Mathematics of Love, my 1976 narrator, Anna, begins her story thus:
It's an early daguerreotype, more than a hundred and fifty years old. That's a positive process, I usually explain, the plate exposed directly so that each is unique... If you tilt it towards the light, the image gleams and shimmers, so exact, so bright and so dark, that the moment of its taking seems to live inside the glass. The sun touches the pillars and chimneys of Kersey Hall and flickers among the dark, late-summer trees. Its light lies on the lawn, strokes the curve of the steps, slips through the half-open door to where a figure in a long dress stands. It was then - that moment - that the shutter opened, and snatched a scatter of the light and dark, throwing it on to this piece of glass, fixing the sun and shadow of those few seconds for ever. And the sun moved on and took the day with it, while the plate held those shadows and kept them, and carried them to other places and other times before it was found again. One of those times was mine.
And later in the novel, although much earlier in Anna's life, veteran photographer Theo - who I modelled on Robert Capa - says this:
"...all photographs are about death, really, and time: about preserving a moment in silver and chemicals, when life itself is never preserved, when every cell of everything is already decaying, and being replaced, and decaying again. The subject and the image co-exist for the moment that the shutter opens and closes. And then the subject decays but the image lives on unchanged."
"It makes it sound like a ghost."
He grinned at me. "Oh, Anna, does the idea give you gooseflesh all over?... but a photograph does prevent something - somebody - from being laid to rest."
I moved a little, and felt the slight, hot roughness where my shoulders were newly brown. I looked down and saw that the image of the dark straps of my top was printed pale by the sun.The Mathematics of Love is all about ghosts, without really being a ghost story. And if I told you what I mean by "not really" it would spoil things, but the more I thought about light and where it falls, the more places I found - such as Anna's sun-tan - where it's part of how we experience time and our selves. I don't believe in ghosts - hence my "not-really". But I think what ghosts embody and express about our sense of the past is endlessly fascinating - and not just the past, but the dead and the still living and the might-have-been. And in that way, a photograph is like a historical novel as much as it's a historical record; it embodies the dead quite literally, in silver halides, in a way which lives absolutely in our moment. As A L Berridge was saying, it therefore embodies the might-have-been, just outside the frame. It's more like holding a letter written by that person, than it's like looking at a portrait painted of them. Writing historical fiction is all about using imagination (rooted in research, of course) to cross over from the present into the past. With a photograph, the past crosses over into our present.