Filter factor
In photography, filter factor refers to the multiplicative amount of light a filter blocks. Converting between filter factors and stops. The table below illustrates the relationship between filter factor, the amount of light that is allowed through the filter, and the number of stops this corresponds to. Calculating exposure increase. The number of f-stops of light reduction, given a filter factor, may be calculated using the formula: Most calculators do not have a formula_2 function. An equivalent calculation is: or An example: A green filter with a filter factor of 4 The green filter factor of 4  yields a 2 f-stop light reduction. The filter factor, given the exposure change in f-stops, may be calculated using the formula: An example: A deep red filter with an f-stop change of 3 stops A change of 3 f-stops is equivalent to a filter factor of 8. As a consequence of this relationship, filter factors should be "multiplied" together when such filters are stacked, as opposed to stop adjustments, which should be added together. Filter factors for common filters. The table below gives approximate filter factors for a variety of common photographic filters. It is important  to note that filter factors are highly dependent on the spectral response curve of the film being used. Thus, filter factors provided by the film manufacturer should be preferred over the ones documented below. Furthermore, note well that these factors are for daylight color temperature (5600K); when shooting under a different color temperature of ambient light, these values will most likely be incorrect.