The B + W filters are made of brass alloy with excellent physical properties and high-purity, distortion-free, thick Schott glass, with a particularly precise design, state-of-the-art manufacturing technology and the highest quality standards. Finally, different types of surface treatment are provided to complete the perfection of B + W filters. From the raw materials used to the final touches, 100% "Made in Germany". A neutral gray filter is used to reduce the amount of light entering. Its use is necessary if e.g. In photography, we want to use a longer shutter speed with low depth of field (wide aperture) (swinging or veiling moving water) and get the most overexposed images at the lowest sensitivity. In video recording, shooting is too bright in bright weather with the average refresh rate (30 fps) and wide aperture (subject enhancement). You can choose from several filters that result in different intensities of light reduction. The B + W ND4 gray filter (factory code: 102) causes a reduction of 2 fps (allowing a quarter of the light into the lens), e.g. with the same sensitivity and shutter speed (image refresh in video), you can work with an aperture that is 2 apertures wider (such as f / 2 instead of f / 4). This filter is equipped with eight layers of surface treatment (MRC) on both sides, which provide anti-reflection and dirt resistance, among other things. This filter is made with an F-pro socket, which is a standard B + W socket that is also great for use with wide-angle lenses. They are also equipped with a front thread, so that an additional filter (eg CPL) or accessory can be attached, as well as internal nail lens caps.