Blue filters are used for colour conversion when exposing daylight-balanced film in artificial light (Hoya 80A, B and C), and to increase colour temperature when slightly cooler tones are required (Hoya 82A, B and C).
- An 80A filter increases colour temperature from 3200oK to 5500oK for use with 3200oK lamps;
- an 80B filter increases colour temperature from 3400oK to 5500oK for use with Photoflood lamps;
- an 80C filter increases colour temperature from 3800oK to 5500oK for use with clear flash bulbs;
- an 82A filter provides a small increase in colour temperature for slightly cooler tones;
- an 82B filter should be used when exposing tungsten type B film (3200oK) under tungsten domestic lamps (2900oK). It may also be used in portraiture to eliminate reddish skin tones in early morning and late afternoon; and
- an 82C filter provides a larger increase in colour temperature for more noticeable cooler tones.
82-series filters should not be mixed with the 81-series warm-up filters as each cancels the effect of the other.
The filter factor for an 80A filter is generally 4 x. For an 80B filter it is generally 2.75 x, or 1.7 stops.