ARRI’s new FSND external filter.
ARRI has released a larger version of their Full Spectrum Neutral Density (FSND) filter in 6.6”x 6.6” and 4”x 5.65” sizes for use in external matte boxes. ARRI is already very familiar with premium filters; the ALEXA Mini and the AMIRA cameras come equipped with a colour-neutral internal FSND filter. FSND filters reduce the amount of light entering the lens, allowing the filmmaker to shoot at wider T-stops under bright conditions without overexposure. FSND filters also make it possible to blur moving elements such as water or traffic in bright conditions.
The optical quality, Schott B270i glass used in the ARRI FSND filter is manufactured to a very high standard. The glass is polished completely flat on both sides of the filter with completely parallel surfaces. Plane parallelism is especially important when using a wide-open telephoto lens, as areas of the focal plane can become soft.
Not only have the filter materials and shape been expertly selected, but ARRI has devoted a great deal of time and energy to the various coatings of the ARRI FSND filters. Multiple layers of ND coating, anti-reflective (AR) coating, hydrophobic coating, oleophobic coating, as well as a final “hard” coating have been applied to both sides of the filter.
The normal 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 is a sensation. Reducing reflectivity maximises contrast, allowing the full dynamic range of the sensor to be captured, facilitating HDR workflows.
The additional coatings, hydrophobic and oleophobic, keep water and grease, respectively, from adhering to the surface of the glass, thereby cutting down on cleaning time dramatically.
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