80A filters increase color temperature of the light from 3200K to 5500K and correct the use of daylight film with standard household incandescent lamps. They have a filter factor of 2.4 and require 1 1/3 f-stops of additional light.
All light sources, natural or artifical, have characteristics that affect color rendition on film. Most films are engineerd for either daylight or tungsten photofloods which are rated respectively at 5500K degrees (daylight) and 3400K degrees (photofloods) on the Kelvin(K) color temperature scale.
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) are approximately 5500K and will 'overpower' household lighting. With daylight film, no filtration is necessary unless special effects are desired.
80 series filters come in several strengths are available in most sizes.
Other 80 series filters are:
80B for use with tungsten photofloods.
80C for use with clear flash bulbs.
Other useful color correcting filters are:
85 series for tungsten film in daylight; The opposite of 80 series.
81 series for warming effect.
82 series for cooling effect; the opposite of 81 series.
FL-D (daylight) when using daylight film in florescent lighting
FL-B (bulbs) when using tungsten film in florescent lighting.