Filter factor

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 required to correct the exposure with a given filter may also be calculated using the formula:

where the exponent "X" is the number of f-stop increases required.

An example: A deep red filter with a filter factor of 8

The normal exposure will be increased by three stops with this filter. 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.