A number that indicates to what extent you must increase exposure when you use a particular filter (by multiplying the unfiltered exposure by the filter factor number). Filters absorb light. The filter factor allows you to compensate for this absorption. The amount of exposure compensation has been predetermined for every filter, and is expressed as a "filter factor" (sometimes also called an exposure factor, and also referred to as Exposure Magnification or EM values).

Number by which an unfiltered exposure reading must be multiplied to give the same effective exposure through the filter. This compensates for the absorption of light by the filter. This process is unnecessary with TTL metering systems as long as the filter is attached during the metering process.

In photography, filter factor refers to the multiplicative amount of light a filter blocks.