Neutral Density (ND) filters appear gray in color and reduce the amount of light reaching the camera's sensor. Since the transmission value only varies over a small percent in the visible, there is no effect on color balance. Instead of just stopping down the lens aperture due to high light levels, ND filters allow for wider apertures which can be used to decrease the depth of field. This allows the important information to be separated from the background. Compared to polarizing filters, ND filters are ideal for overall light reduction in cases of extreme light intensity. By stacking filters, it is possible to achieve other Optical Density (OD) values. All mounted filters can be threaded together with identical male and female threads on each mount. Optical Density exhibits an additive relationship; for example, stacking filters with OD values of 0.6 and 0.9 yields a resultant density of 1.5. The optical density is related to the transmission by the following equation: T = 10-D x 100 = percent transmission.