A contrast filter passes some wavelengths and absorbs others, so it always reduces the total light reaching the film. Left uncorrected, that loss underexposes the negative. The filter factor is the figure that quantifies the loss and tells how much exposure to add back.

What the Factor Represents

A filter factor is a multiplier applied to the unfiltered exposure. A factor of 2 means the filtered scene needs twice the exposure to place the same density on the negative; a factor of 8 means eight times. The number describes only light lost to absorption, not the contrast effect the filter produces.

Factors are derived photometrically: matched filtered and unfiltered exposures are made of a constant source, and the ratio of exposures required to reach the same density is the factor. Because that figure depends on what the film “sees,” it is published per emulsion. The Wikipedia reference on filter factors notes that values “are highly dependent on the spectral response curve of the film being used,” so a single published number is only an approximation for any given stock.

Why It Shifts With the Source

A filter factor is valid only for the spectral content of the light used to measure it. Tungsten light is far richer in red and poorer in blue than midday daylight, so a filter that absorbs blue throws away less of a tungsten source. A red filter illustrates this clearly: Kodak’s data for the Wratten No. 25 (red tricolor) lists a factor of 8 in daylight but 5 in tungsten. The same glass, the same film, a different number — because the source spectrum changed. Conversely, a blue filter costs more exposure under tungsten than under daylight.

Converting to Stops

A stop is a doubling of exposure, so the conversion is logarithmic: stops equal the base-2 logarithm of the factor. A factor of 2 is one stop, 4 is two, 8 is three, and intermediate values fall between — a factor of 5 is roughly 2 1/3 stops. A medium yellow Wratten No. 8 at a factor of 2 therefore asks for one stop, a No. 25 red at 8 for three.

When two filters are stacked, the factors multiply while the stops add. Adjusting shutter speed scales linearly with the factor; adjusting aperture moves by the stop count. Through-the-lens metering reads the filtered light directly, but where the meter’s spectral sensitivity diverges from the film’s, the published factor remains the more reliable correction.