A gradient filter is an optical filter that is dark at the top to reduce the intensity of light coming through it and clear at the bottom to allow it to pass through unchanged. Also known as a graduated neutral density filter, it helps photographers shoot bright subjects such as the daytime sky without their images being over exposed.
Purpose
Every camera operates within a range of brightness known as a dynamic range. Objects that are brighter than the maximum light level in this range will cause over exposure in the final image. To avoid this, photographers can place gradient filters over their camera lenses to reduce the intensity of the bright areas while leaving the other areas alone. This allows photographers more options in terms of their aperture, shutter speed and ISO levels as they can adjust these settings based on their artistic preference without worrying about over exposure.
Types
There are two basic types of gradient filter. The first gradually fades from dark to clear, while the second changes its density more abruptly. Gradient filters with gradual fades are said to have a "soft edge." Filters with less gradual fades are said to have a "hard edge." In practice, most gradient filters have a soft edge to some extent.
Uses
Photographers generally use gradient filters to shoot landscape images during the day. Soft-edged gradient filters are useful when the scene being photographed starts off bright at the top and gradually gets darker toward the bottom, such as in the case of shooting horizon with a diffuse tree line. In situations where the brightness of the subject changes suddenly and dramatically, such as in the case of shooting the horizon over the ocean, harder edged gradient filters are more useful.
Considerations
The advent of digital photography has led to the creation of image editing tools that allow photographers to selectively adjust the brightness of images during the post-production process. However, if there's too much light present at the time a digital image is taken, it may cause "clipping." This is when the light is so intense that it actually results in data loss. While there are ways of masking this in post production, there is no way to eliminate the effect entirely.
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