Warming filters reduce excess blue tones that effect daylight film when shooting sky or sea scenes or in high altitude photography. They are also effective if your flash exposures are too 'cool'. 'A' is the mildest of the 81 series. It has a filter factor of 1.4 and requiring 1/2 f-stop of additional light.
All light sources, natural or artifical, have characteristics that affect color rendition on film. Color correcting filters are designed to 'warm', 'cool' and/or correct for the many variations of lighting you might encounter.
All filters have a 'filter factor' which is a measurement of the light it absorbes. Cameras with internal metering (through the lens) will automaticly adjust the exposure. With external or hand held meters you must allow for this factor.
Note: Modern strobes (flash) approximate midday sun and any warming or cooling filtration with flash should be based on 'daylight'.
81 series warming filters come in several strengths and are available in most sizes.
Other 81 series filters are:
81B slightly stronger than 81A.
81C Twice the warming effect of 81B
81EF extremely strong warming effect.
Other useful color correcting filters are:
82 series for cooling effect: opposite of 81 series.
80 series when using daylight film in tungsten lighting.
85 series when using tungsten film in daylight.
FL-D (daylight) when using daylight film in florescent lighting.
FL-B (bulb) when using tungsten film with florescent lighting.