I am attempting to work out, conceptually, how the use of a neutral density filter effects the resulting captured image.
Do ND filters allow a camera to capture higher contrast scenes (without clipping the highlights) than would otherwise be possible? Or do they just decrease the amount of light uniformly so that highlight detail is gained at the expense of shadow detail being lost?
Put more technically, is the effect of a ND filter more like a linear mapping or a clipping of the low light levels? See the figure below for examples of each.