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When it is desirable to maintain a particular lens opening for sharpness or depth-of-field purposes, or simply to obtain proper exposure when confronted with too much light intensity, use a neutral density (ND) filter. This will absorb light evenly throughout the visible spectrum, effectively altering exposure without requiring a change in lens opening and without introducing a color shift.
Neutral density filters are denoted by (Optical) Density value. Density is defined as the log, to base 10, of the Opacitance. Opacitance (degree of absorption) of a filter is the reciprocal of (and inversely proportional to) its Transmittance. As an example, a filter with a compensation of one stop has a Transmittance of 50%, or 0.5 times the original light intensity. The reciprocal of the Transmittance, 0.5, is 2. The log, base 10, of 2 is approximately 0.3, which is the nominal density value. The benefit of using density values is that they can be added when combined. Thus two ND .3 filters have a density value of 0.6. However, their combined transmittance would be found by multiplying 0.5 x 0.5 = 0.25, or 25% of the original light intensity.
Neutral density filters are also available in combination with other filters. Since it is preferable to minimize the number of filters used (see section on multiple filters), common combinations such as a Wratten 85 (daylight conversion filter for tungsten film) with a ND filter are available as one filter, as in the 85N6. In this case, the two stop ND .6 value is in addition to the exposure compensation needed for the base 85 filter.
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