Graduated neutral-density filter

Graduated filters were used in the early twentieth century, for example for darkening skies in landscape photos.

[2] One advertised brand was called the "Lifa graduated filters for sky, cloud, and landscape photography".

[4] In a discussion at the Royal Photographic Society in 1910, some "debatants" held the use of such "graduated color screens" to be quite limited.

"[6] Although its importance may have lessened with the advent of the modern digital darkroom, graduated ND filters are still an important tool for professionals because a digital sensor that is clipping ("blown out" or "washed out") captures no usable data in the clipped area, an effect which cannot be corrected with later processing because data has been lost.

As an alternative to split graduated neutral density filters, some digital cameras offer built-in high-dynamic-range imaging (HDR) which allow the camera to capture and then combine different exposures of the same subject matter when shooting in RAW image format.

A GND filter held up to the horizon. Note the poor contrast in the overexposed part of the sky not covered by the filter.
The same image with washed-out (white) pixels colored red. Note that clouds that are washed out without the GND filter show detail behind the filter.
The same image with a GND-like effect applied to the right side of the image using a computer. While the overall effect is similar, regions of the image that were washed out are not recovered.
A Cokin 3-stop (ND8) graduated ND filter.
Stacked cases of Cokin filters.