Performance across the spectrum.
The color cast seen in most neutral density filters on the market can be seen on a spectral graph as deviations from a flat curve - usually a blue cast that results in an increase in % transmission from 400 - 500nm. The X4 ND has the flattest transmission curve throughout the visible range and well into infrared, leading to a more color neutral performance.
And thanks to years of research and development, the X4 ND is incredibly precise to the ideal optical densities. An optimal 10-Stop (3.0) ND filter has a light transmission of 0.1%. With an average light transmission of 0.108% across the visible spectrum, the X4 ND hits the mark with great accuracy giving you a more dependable baseline to set up shots.