The so-called warm-up filters, such as the Hoya 81A, B and C, are used to achieve small decreases in colour temperature. This produces a warmer feeling in a colour image, and counteracts any tendency towards bluer tones. The Hoya 81A should be used when exposing tungsten Type B film (3200oK) with Photoflood lamps (3400oK).
The filters may be combined to achieve various decreases in colour temperature, but in many cases the Hoya 81B may prove effective.
The Hoya 85-series filters (A, B and C) are colour conversion filters used when exposing tungsten-type colour films in daylight.
- An 85A filter decreases colour temperature from 5500oK to 3400oK for the use of Type A colour film;
- an 85B filter decreases colour temperature from 5500oK to 3200oK for the use of Type B colour film; and
- an 85C filter decreases colour temperature from 5500oK to 3800oK.
The filter factor for 85-series filters is generally 1.5 x. For an 81A the filter factor may be considered to be zero.