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.