One tall window

What you are actually dealing with

A north-facing sash window on an overcast afternoon. A subject pulled close enough that the light starts to fall away across the far cheek. That is, in its essentials, the same arrangement that portrait painters worked with for several centuries before a camera existed, and the reasons it works have not changed: the light is large relative to the subject, it comes from one direction, and it moves across a face slowly enough that small adjustments matter.

The difficulty is that the light is in charge, not you. Window light shifts with cloud, with the hour, with a lorry passing in the street outside. What reads as a workable shadow on one side of the face can tip into a block of nothing inside three minutes. Falloff — the rate at which light diminishes with distance — is the whole problem: a subject a metre from the glass is in a different picture from one two metres back. Moving a chair twelve inches can be the whole difference between a portrait that lives in the shadows and one that does not.

Bill Brandt understood this materially — his English interior portraits from the 1930s and 1940s carry the weight of rooms in them, and the light is clearly domestic, clearly from windows, often cold. The Victoria and Albert Museum holds a substantial part of his archive, and looking at that body of work in sequence you can feel how much of it was made by placing a person in an existing pool of light and waiting for the right moment rather than constructing anything. That patience, that willingness to let the room be the room, is what separates his portraits from studio work of the same period.

The thing that goes wrong

The sitter relaxes and turns slightly away. It feels natural, it is natural, and it puts the near eye in shadow and the far eye in the light, which is rarely what you want. The geometry of a window portrait is unforgiving of small drifts: a face that is slightly too square-on to the glass looks flat; a face turned too far away loses the modelling and the light falls on an ear. The working position is narrow.

Then there is the background. A window portrait made without thought about what is behind the subject is often a portrait of a curtain, or a wall at the wrong tone, or a doorframe cutting into a shoulder. The room is part of the picture even when you cannot see it clearly, and the subject's tonal relationship to it — lighter or darker, how much separation — is a decision that has to be made before the sitting starts, not recovered afterwards.

Eve Arnold, who worked for Magnum Photos, routinely worked with available light in confined domestic spaces. What the contact sheets of that kind of work reveal — the frames before and after the ones that made it — is how much is about small corrections: the head angle shifting, the window light catching or losing an eye, the background drifting in and out of a useful tone.

The older craft answer to the falloff problem is a reflector on the shadow side — something white, something that bounces the existing light back. This is not a technical intervention so much as an act of listening to what the light is already doing and helping it reach a little further. The risk is that it softens the portrait into something bland: the shadow on the far side of a face is often where the character is, and filling it flat takes that away.

A tall window, overcast sky, a chair close enough to the glass. The same constraints as always, and they still produce something that other kinds of light do not.