#Neutral Density Filter
A neutral density or ND filter is a filter that is used to darken or lower the amount of light that passes through the lens by a specified number of stops, ranging from a fraction of a stop to 10 stops or more. There are also variable neutral density filters that change the amount of light allowed to enter when rotated. Uses for neutral density filters include shooting in bright light with a fast aperture for shallow depth of field, capturing video at appropriate shutter speeds in bright light, and taking long exposures during the day.
Neutral Density Filters Explained
A neutral density filter is, ideally, a neutral grey filter that has no effect on the image other than darkening it. (As opposed to UV filters, Polarizing Filters, and other special effects filters that may significantly influence the nature of the resulting image.)
Ideally, a good neutral density filter will not introduce any color cast to an image, or reduce the image detail / sharpness by any discernible amount. Unfortunately, some ND filters, from the affordable to the expensive, do introduce a faint color cast to the resulting image.
While a warm or cool side effect can easily be compensated for by adjusting the camera's white balance, unfortunately the color issues with ND filters also manifest in not just temperature but also tint, or the magenta / green spectrum. This can still be roughly corrected for either in-camera or in post-production, however the purity of the overall subtle colors may still be compromised.
If a photographer only finds themselves using an ND filter very rarely, then a "cheap" one is likely good enough to achieve the desired effect. However a photographer or videographer who uses ND filters on a regular basis, let alone multiple ND filters of different densities or sizes, would be wise to invest in a filter set that is as color-neutral as possible, and also consistent between each filter.