Low light generally equals high ISO and so forth. But for a portrait, presumably you are in control of all the elements of a photo, and that means control of the light. Not just the exposure for the light, or the amount of light, but the essence and expression of the light—quality, color, and direction. When you have command of those aspects of light, you are speaking with it and not just exposing for it. Jacking up your ISO is not an interpretative act on the part of the photog. More often, it’s an act of desperation.

  Extrait de: Joe McNALLY Sketching Light: An Illustrated Tour of the Possibilities of Flash.