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.
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.
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.