The 4 x 4" Neutral Density Attenuator 1.5 Filter from Schneider is a unique filter for the still or MPTV shooter. Unlike traditional graduated ND filters that are 50% clear and 50% ND with a certain density factor, Attenuators are graduated over the entire surface of the filter. For example, the 0.3 (1 stop) attenuator will be clear glass at the bottom of the filter and 1 stop ND at the top with a gradual progression throughout the remainder - with probably a 1/2 stop ND somewhere in the middle - but the major difference between the two types of filters is that the transition in Attenuators is seamless.
With traditional ND filters, the transitional line in the center of the filter can either be soft edge (smooth transition) or hard edge (much more pronounced) but maximum ND is quickly achieved shortly after that horizon line and remains constant for the rest of the filter. Neutral Density grads are perfect for scenes that have a definite horizon where you can place the ND edge on; Attenuators are much more subtle and are better suited for situations that don't allow you to hide that edge - perhaps a street scene that needs to be balanced where one half is lit by sunlight and the other half is in shadow.
Schneider Attenuators are made from Water White glass and characterized by a high-transmission quality. Typically, it will transmit 98-99% of the light. It also features an anti-glare and anti-reflective quality that makes it useful in a wide range of uses including the movie theater industry, store and art displays. The glass is virtually defect-free and is becoming more and more prevalent in digital equipment.