PURE Harmonie Variable Neutral Density X filters are the lightest and thinnest in the world measuring in at only 9.5mm. They have been engineered with no compromise; very robust, compatible with all lens caps, and multi-coated with EVERCLEAR 5 Coating Technology to withstand almost any conditions. The PURE Harmonie ND X filter is the result of extensive engineering. Its form factor has been designed so that the external diameter of the filter is wider than the front of the lens to avoid vignetting issues that may occur with extreme wide angle lenses. Neutral density filters are used to reduce the quantity of light that reaches the sensor of the camera. They allow the photographer to decrease their shutter speed in order to achieve motion blur (waterfalls, ocean, clouds....) and/or reduce depth of field in very bright conditions. As they are "neutral", ND filters do not affect color renditions. With PURE Harmonie Variable Density Neutral Gray filters, users can select the light reducing intensity from ND2 (1 f-stop) to ND400 (8+ f-stops) by rotating the front ring. While some variable ND filters can become yellowish when increasing the density, PURE Harmonie ND X filters remain perfectly neutral. They are PURE.