A landscape under a bright sky routinely spans a brightness range wider than film or sensor can record in a single exposure. The sky may sit four or more stops above the shadowed land beneath it. Expose for the foreground and the sky records as featureless white; expose for the sky and the land falls into empty shadow. A graduated neutral density filter addresses this at the moment of capture by darkening only the brighter portion of the frame, compressing the scene’s luminance range to fit within the recording medium’s usable latitude.

How a Graduated Filter Compresses the Range

A graduated ND filter is a rectangular sheet, dense at one end and clear at the other, with a transition zone between. Positioned so the dense half covers the sky, it holds back light there while passing the foreground unattenuated. Because the density is neutral, it absorbs across the visible spectrum more or less evenly and does not shift colour or tonal rendering, unlike a coloured filter.

Density is specified as optical density, defined as the base-ten logarithm of the reciprocal of transmittance. One stop of light loss corresponds to a density of 0.30, since each stop halves transmitted light and log10(2) is approximately 0.30. Manufacturer ranges follow this scale directly: Lee Filters lists its neutral density grads in steps of 0.3 (one stop), 0.6 (two stops), 0.9 (three stops), and 1.2 (four stops), among intermediate values. Selecting a filter is therefore a matter of metering the difference between sky and foreground and choosing a density that brings the two within range.

Hard, Medium, and Soft Transitions

Filters differ not only in density but in how abruptly the dense region gives way to clear glass. A hard-edge grad changes over a narrow band, suiting scenes with a flat, unbroken horizon such as open sea or level plains, where the line dividing bright sky from darker land is sharp and can be aligned precisely. A soft-edge grad spreads the transition across a broad zone, blending more forgivingly over irregular horizons broken by mountains, trees, or buildings, where a hard line would leave visible darkening on anything rising above it. Medium grads occupy the ground between. Lee Filters offers soft, medium, hard, and very hard variants on the same density values for this reason.

Why Focal Length Alters the Apparent Edge

The transition does not record as sharply as it is etched on the filter, and how soft it appears depends on the optics. A graduated filter sits in front of the lens, far from the plane of focus, so its edge is rendered out of focus and spread by the aperture. A wide-angle lens, with its great depth of field, renders even a hard grad as a relatively gradual gradient; a telephoto lens renders the same edge much more distinctly. A wider aperture further blurs the transition, a smaller one sharpens it. The choice between hard and soft therefore cannot be made from the horizon alone: one scene may call for a hard grad on a long lens and a soft grad on a wide one.

Holding back the sky in this way records detail in both extremes within one frame, leaving the negative or file with a luminance range that ordinary processing can carry.