ARRI Full Spectrum ND External Filter Preserves Dynamic Range


ARRI has developed its own full spectrum neutral density (FSND) external filter. Its design is based on the filters built into ARRI's ALEXA Mini and AMIRA cameras, which have internal FSND filters that are entirely colour neutral to preserve the camera sensors' dynamic range.

The external FSND filter comes in 6.6in x 6.6in and 4inx 5.65in sizes and is made of optical quality Schott B270i glass. The glass is polished completely flat on both sides of the filter, giving it precisely parallel surfaces. Parallel planes are especially important when using a wide-open telephoto lens on the camera, which can cause areas of the focal plane to appear soft. FSND filters reduce the amount of light entering the lens, allowing the filmmaker to shoot at wider t-stops (light transmission values) under bright conditions without overexposure. FSND filters also make it possible to blur moving elements such as water or traffic in bright conditions.

Several layers of ND coating, anti-reflective coating, hydrophobic coating, oleophobic coating and a final hard coating have been applied to both sides of the filter. A typical air-facing filter has a 4% - 6% reflectivity rating at each air-glass surface, but ARRI has been able to achieve 0.2% reflectivity. Since crews often stack filters together and reflectivity is cumulative, reducing this aspect down by a factor of 20 makes a difference. Reducing reflectivity maximizes contrast, promoting capture of the full dynamic range of the sensor to support HDR workflows.

The hydrophobic and oleophobic coatings keep water and grease, respectively, from adhering to the surface of the glass, to cut down on cleaning time. The hard coating, used to prevent scratches on the filter and extend its life, is very durable and resistant to wear. Filters need to withstand repeated rubbing or cleaning in the same area without leaving a mark.

The edges of the ARRI FSND filters are c-shaped with a curved edge to reduce chipping of the glass and make mounting easier and safer. The edges are also coloured black to prevent light from scattering. Close to the filter’s edge, the glass surface is engraved with a unique Direct Product Marking (DPM) as a security measure for individual owners and rental house inventory tracking. The mark is a GS1 bar code, readable by most camera-based bar code readers around the world, and makes rental check-ins and check-outs easier, serves as proof of ownership and means stickers are unnecessary.

The filters come with protective pouches made of water-resistant CORDURA outside and very soft, anti-static microfibre-like material called MICRODEAR inside that cleans the surface of the filter each time it is removed or replaced.