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The Tiffen 5.65 x 5.65" Low Light Dispersion Glass Filter is popular with cinematographer's.
The Low Light Dispersion filter (LLD) is the same shade as a traditional yellow #85 filter (The #85 series will produce natural colors when shooting with tungsten film outdoors). However, it is used instead of the #85 in low light scenarios since it does not require the 2/3 stop light loss of the #85.
This filter opens up shadows while adding a noticable warmth to the image.