Graduated Neutral Density 4x 77mm Filter
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This graduated neutral density filter is used to reduce contrast in the foreground or background. Half of the filter is dark, and the other half is clear, so you can choose what you would like to darken in a scene. This 4x filter reduces the amount of light coming into the lens by 2 stops.
These filters are very popular for sunsets, where you need to keep the shutter open long enough to expose the foreground without overexposing the background.