Digital Video and Film Filters
Filters intend to change the way that the digital video or film camera ultimately reads and prints the image. It can do a number of different things according to its set purpose, as well as be secured to the lens externally with an adaptor ring or go behind the lens. A neutral density filter is actually one of the more popular and standard filters for digital video use.
Neutral density filters, usually called ND filters, are filters that actually interrupt all colors in the given frame because of their grey appearance. The over all purpose of the ND filter is to cut down on light in a situation without causing too much impact on color quality.
When light is reduced color tends to change. The ND filter counters that by creating a filter that changes all colors inherently, which preserves them as light is reduced. With the ND filter you can change things like aperture and depth of field and still cut down the light in the way that you want.
For example, if you want to open up the aperture in an effort to reduce the over all depth of field the ND filter will help you cut down on the increase of light. In the end the neutral density filter is just a method to cut down on light if it is too bright in your given situation.
ND Filter Measurement
The ND filter is measured in a specific increment. This increment equals to 0.1 ND. 0.3 ND will equal a single stop. As you stack up the ND filters you will just add these numbers to estimate the stops. Look at the ND filters and you will see this measurement being marked to find out how many stops it represents.
Using ND Filters
ND filters are used quite often in conjunction with other filters, though this can often increase video noise depending on what combination is being used. This is also going to be tempered if you are using actual film stock and not digital video. In this case the type of film stock that you are using is going to play a role in your filter choices. This is true not of just neutral density filters, but all filters. You will often find that there are some built in ND filters on some digital video cameras that you can turn on or off in different capacities.