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ND filters put HDR in contrast

Arri has released its first external filter, with its new Full Spectrum Neutral Density (FSND) filter range. Designed for the Alexa Mini and Amira, they are claimed to offer the “highest contrast
and true colour transmittance for HDR capture”.

The two cameras already come with an internal FSND filter that is purely colour neutral, and Arri wanted to offer external filters of the same quality. The Arri FSND filter comes in 6.6×6.6-inch and 4×5.65-inch sizes, using Schott B270i glass polished completely flat on both sides of the filter with completely parallel surfaces.

This is important when using a wide-open telephoto lens, as areas of the focal plane can become soft. There are eight filters, ranging from 0.3 to 2.4 ND. Unlike most external filters, which have square edges, the curved edges of Arri’s FSND filter reduce chipping and make for easier mounting.

The filter’s edges are also black to prevent light scatter.

An engraved barcode makes inventory management easier.

Multiple layers of ND coating, anti-reflective coating, hydrophobic coating, oleophobic coating, plus a final hard coating have been applied to both sides of the filter (something only offered by Arri). A normal air-facing filter has a 4-6 per cent reflectivity rating at each air-glass surface, but Arri has achieved 0.2 per cent reflectivity. Since users often stack filters together and reflectivity is cumulative, this makes a big difference to contrast, particularly for HDR.