Glossary
Filter factor
How many stops of light a colored, ND, or polarizing filter absorbs.
A red filter on black and white film typically costs around three stops. A polarizer costs about one and a half to two stops. A 3-stop ND filter costs three stops. The meter cannot see the filter through the lens, so you tell it which filter you are using and the app subtracts the loss from the recommended exposure. Without filter compensation, every reading through a filter would be wrong.
