Filter factor is a numerical value assigned to an optical filter that indicates by how much the camera's exposure settings must be increased to compensate for the reduction in light caused by placing the filter in front of the lens. Because optical filters absorb or block a portion of the light passing through them, the amount of light reaching the film or image sensor is reduced, and without compensation this would result in an underexposed image.
The filter factor is expressed as a multiplier - for example, a filter factor of 2x indicates that the exposure should be doubled compared to the unfiltered reading, equivalent to an increase of one stop. A filter factor of 4x requires two additional stops of exposure, and an 8x filter factor requires three additional stops. As a general rule, each time the filter factor doubles, one additional stop of exposure compensation is required.
In practice, exposure compensation can be applied by adjusting the aperture, the shutter speed, or the ISO setting, depending on the creative and technical requirements of the shot. Many modern cameras with through-the-lens (TTL) metering systems automatically account for the light reduction caused by a filter, as the meter reads the light after it has already passed through the filter, effectively applying the necessary compensation without any manual calculation. However, when using a handheld light meter or calculating exposure manually, the filter factor must be applied by the photographer.
Filter factors are typically printed on the filter itself or included in the manufacturer's documentation, and can vary depending on the type and density of the filter as well as the colour temperature of the light source being used. Polarising filters, for example, have a variable filter factor that changes depending on the angle of the filter relative to the light source.