中性灰度镜(ND Filter),减光镜，光学衰减片

         中性灰度镜又称中灰密度镜（有的俗称为 灰玻璃）、减光镜、光学衰减片等，ND FILTER，英文全称Neutral Density FILTER，简称ND镜，是一块灰色透明的光学玻璃。其作用就是通过衰减进入镜头的光线强度，而对各种波长的光线衰减程度近似相同，即非选择性衰减，也就是说，ND镜对各种不同波长的光线的减少能力是同等的、均匀的，只起到减弱光线的作用，而对原物体的颜色不会产生任何影响，因此可以真实再现景物的反差，彩色摄影和黑白摄影同样适用。

            Neutral Density Filter, also known as medium gray density mirror, ND FILTER， The full English name is Neutral Density Filter, abbreviated as ND Filter, which is a gray transparent optical glass. Its function is to attenuate the intensity of light entering the lens, and the degree of attenuation for various wavelengths of light is approximately the same, that is, non selective attenuation. That is to say, ND mirrors have the same and uniform ability to reduce light of different wavelengths, only playing a role in reducing light without affecting the color of the original object. Therefore, it can truly reproduce the contrast of the scene, and color photography and black and white photography are also applicable.

          根据衰减光线能力的强弱，中性灰度镜有多种密度可供选择，如ND2、ND4、ND8，也可以多片中性灰度镜组合使用。不难看出，ND后面的那个数字，即代表了ND镜衰减光线的能力。

          According to the strength of its ability to attenuate light, neutral grayscale mirrors have multiple densities to choose from, such as ND2, ND4, ND8, and can also be used in combination with multiple neutral grayscale mirrors. It is not difficult to see that the number after ND represents the ability of ND mirror to attenuate light.
         使用ND镜主要的目的是防止过曝，如果光线太亮就很难选择较慢的快门速度拍摄，这时使用ND镜减少进入镜头的光线，就能够使用较慢的快门拍摄了。例如，如果需要在阳光强烈的室外拍摄，又或者需要在正常光线条件下用较长的曝光时间，以慢速快门拍摄瀑布以表现出虚化的水流等特殊效果，都需要ND镜。

        The main purpose of using an ND lens is to prevent overexposure. If the light is too bright, it is difficult to choose a slower shutter speed for shooting. In this case, using an ND lens to reduce the amount of light entering the lens allows for shooting with a slower shutter speed. For example, if you need to shoot outdoors in strong sunlight, or if you need to use a longer exposure time and slow shutter speed to capture waterfalls under normal lighting conditions to show special effects such as blurred water flow, you need an ND Filter.

ND Filter常用光谱曲线(ND Filter commonly used spectral curves)：