Neutral gray mirror, also known as the gray density mirror, referred to as ND mirror, is a piece of gray, pure and transparent advanced optical glass. Its role is by weakening through the lens to reduce light exposure. This filter action is a non selective. That is to say, ND filter of different wavelengths of light for their ability to reduce is equal, uniform, weaken the light only played the role of, and color of the original object does not produced any effect, so it can be true representation of scene contrast and color photography and black and white photography apply equally. According to the "block" of light intensity, neutral gray mirror has multiple densities are available, such as ND2, ND4, nd8 (respectively on the extension of a file, file 2 and 3 file shutter speed) can also be more neutral gray mirror used in combination. It is not difficult to see that the figure behind the ND, that represents the ability to block the ND mirror light. The larger the value of ND, the more serious partial color, the general quality of products are using other techniques for wavelength, as far as possible to minimize the distortion of color and the most faithful to restore.

ND mirror only reduces the intensity of light, so it does not affect the final image of the color, it will not affect the camera's white balance and automatic exposure. Use the ND mirror, will not be subject to the color and contrast of the object to produce any effect. In addition, since the SLR is carried out by the lens of the camera, so the ND lens will not have any impact on the camera's automatic exposure system.

In short, the need for people to use a slow shutter, or reduce the case of external light, ND mirror is very useful additional lenses. Of course, due to different lens aperture is different, so the ND mirror also has different caliber, in the use of the lens according to the caliber of the choice of different diameter ND mirror.